feat(bb): Use an environment variable to set the transcript URL (#1750)

Related to #1749 

# Checklist:
Remove the checklist to signal you've completed it. Enable auto-merge if
the PR is ready to merge.
- [ ] If the pull request requires a cryptography review (e.g.
cryptographic algorithm implementations) I have added the 'crypto' tag.
- [ ] I have reviewed my diff in github, line by line and removed
unexpected formatting changes, testing logs, or commented-out code.
- [ ] Every change is related to the PR description.
- [ ] I have
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this pull request to relevant issues (if any exist).

circuits/cpp/barretenberg/cpp/src/barretenberg/bb/get_crs.hpp

@@ -8,12 +8,29 @@
 #include <fstream>
 #include <ios>
 
+// Gets the transcript URL from the BARRETENBERG_TRANSCRIPT_URL environment variable, if set.
+// Otherwise returns the default URL.
+inline std::string getTranscriptURL()
+{
+    const char* ENV_VAR_NAME = "BARRETENBERG_TRANSCRIPT_URL";
+    const std::string DEFAULT_URL = "https://aztec-ignition.s3.amazonaws.com/MAIN%20IGNITION/monomial/transcript00.dat";
+
+    const char* env_url = std::getenv(ENV_VAR_NAME);
+
+    auto environment_variable_exists = (env_url && *env_url);
+
+    return environment_variable_exists ? std::string(env_url) : DEFAULT_URL;
+}
+
 inline std::vector<uint8_t> download_g1_data(size_t num_points)
 {
     size_t g1_start = 28;
     size_t g1_end = g1_start + num_points * 64 - 1;
-    std::string command = "curl -s -H \"Range: bytes=" + std::to_string(g1_start) + "-" + std::to_string(g1_end) +
-                          "\" 'https://aztec-ignition.s3.amazonaws.com/MAIN%20IGNITION/monomial/transcript00.dat'";
+
+    std::string url = getTranscriptURL();
+
+    std::string command =
+        "curl -s -H \"Range: bytes=" + std::to_string(g1_start) + "-" + std::to_string(g1_end) + "\" '" + url + "'";
 
     return exec_pipe(command);
 }
@@ -22,47 +39,56 @@ inline std::vector<uint8_t> download_g2_data()
 {
     size_t g2_start = 28 + 5040001 * 64;
     size_t g2_end = g2_start + 128 - 1;
-    std::string command = "curl -s -H \"Range: bytes=" + std::to_string(g2_start) + "-" + std::to_string(g2_end) +
-                          "\" 'https://aztec-ignition.s3.amazonaws.com/MAIN%20IGNITION/monomial/transcript00.dat'";
+
+    std::string url = getTranscriptURL();
+
+    std::string command =
+        "curl -s -H \"Range: bytes=" + std::to_string(g2_start) + "-" + std::to_string(g2_end) + "\" '" + url + "'";
 
     return exec_pipe(command);
 }
 
 inline std::vector<barretenberg::g1::affine_element> get_g1_data(const std::filesystem::path& path, size_t num_points)
 {
     std::filesystem::create_directories(path);
+    try {
+        std::ifstream size_file(path / "size");
+        size_t size = 0;
+        if (size_file) {
+            size_file >> size;
+            size_file.close();
+        }
+        if (size >= num_points) {
+            vinfo("using cached crs at: ", path);
+            auto data = read_file(path / "g1.dat");
+            auto points = std::vector<barretenberg::g1::affine_element>(num_points);
+            barretenberg::srs::IO<curve::BN254>::read_affine_elements_from_buffer(
+                points.data(), (char*)data.data(), num_points * 64);
+            return points;
+        }
+
+        std::ofstream new_size_file(path / "size");
+        if (!new_size_file) {
+            throw std::runtime_error("Failed to open size file for writing");
+        }
+        new_size_file << num_points;
+        new_size_file.close();
+
+        vinfo("downloading crs...");
+        auto data = download_g1_data(num_points);
+
+        write_file(path / "g1.dat", data);
 
-    std::ifstream size_file(path / "size");
-    size_t size = 0;
-    if (size_file) {
-        size_file >> size;
-        size_file.close();
-    }
-    if (size >= num_points) {
-        vinfo("using cached crs at: ", path);
-        auto data = read_file(path / "g1.dat");
         auto points = std::vector<barretenberg::g1::affine_element>(num_points);
         barretenberg::srs::IO<curve::BN254>::read_affine_elements_from_buffer(
-            points.data(), (char*)data.data(), num_points * 64);
+            points.data(), (char*)data.data(), data.size());
         return points;
+    } catch (std::exception& e) {
+        std::filesystem::remove(path / "size");
+        std::filesystem::remove(path / "g1.dat");
+        // We cannot do anything here except tell the user there is an error and stop the cli
+        throw std::runtime_error("Failed to download srs: " + std::string(e.what()));
     }
-
-    std::ofstream new_size_file(path / "size");
-    if (!new_size_file) {
-        throw std::runtime_error("Failed to open size file for writing");
-    }
-    new_size_file << num_points;
-    new_size_file.close();
-
-    vinfo("downloading crs...");
-    auto data = download_g1_data(num_points);
-
-    write_file(path / "g1.dat", data);
-
-    auto points = std::vector<barretenberg::g1::affine_element>(num_points);
-    barretenberg::srs::IO<curve::BN254>::read_affine_elements_from_buffer(
-        points.data(), (char*)data.data(), data.size());
-    return points;
 }
 
 inline barretenberg::g2::affine_element get_g2_data(const std::filesystem::path& path)