Set version from Cargo.toml (#40)

This uses a genrule to read the Cargo.toml and set `CARGO_PKG_VERSION`.
This is picked up by clap to set the version when asked via `--version`.

Fixes #39

BUILD

@@ -7,6 +7,7 @@ rust_binary(
         "//src/builtin:builtin.pb",
         "//src/builtin:default_build_language.pb",
     ],
+    rustc_env_files = [":generate_rustc_env_file"],
     deps = [
         "//src/builtin:build_proto_rust",
         "//src/builtin:builtin_proto_rust",
@@ -23,6 +24,17 @@ rust_binary(
     ],
 )
 
+genrule(
+    name = "generate_rustc_env_file",
+    srcs = [
+        "Cargo.toml",
+        "src/main.rs",
+    ],
+    outs = ["rustc_env_file"],
+    cmd = "echo \"CARGO_PKG_VERSION=$$($(location @rust_host_tools//:cargo) read-manifest | jq -r .version)\" > $@",
+    tools = ["@rust_host_tools//:cargo"],
+)
+
 rust_test(
     name = "unit_tests",
     crate = ":bazel-lsp",

MODULE.bazel

@@ -8,6 +8,10 @@ rust.toolchain(
 use_repo(rust, "rust_toolchains")
 register_toolchains("@rust_toolchains//:all")
 
+rust_host_tools = use_extension("@rules_rust//rust:extensions.bzl", "rust_host_tools")
+
+use_repo(rust_host_tools, "rust_host_tools")
+
 crate = use_extension(
     "@rules_rust//crate_universe:extension.bzl",
     "crate",