diff with previous versions when generating builtin lockfiles

By copying the prior version, we get the same Lockfile diff as one
would for a "regular" lockfile.

Demo:
```
$ pants run build-support/bin/generate_builtin_lockfiles.py -- semgrep
Lockfile diff: semgrep.lock [semgrep]

==                    Upgraded dependencies                     ==

  semgrep                        1.86.0       -->   1.87.0
  zipp                           3.20.1       -->   3.20.2

==                !! Downgraded dependencies !!                 ==

  rich                           13.8.1       -->   13.5.3

==                     Removed dependencies                     ==

  annotated-types                0.7.0
  pydantic                       2.8.2
  pydantic-core                  2.20.1
```

fixes pantsbuild#21388

build-support/bin/generate_builtin_lockfiles.py

@@ -268,6 +268,17 @@ def generate_jvm_tool_lockfiles(tools: Sequence[JvmTool], dry_run: bool) -> None
 
 
 def generate(buildroot: str, tools: Sequence[Tool], args: Sequence[str], dry_run: bool) -> None:
+    def lockfile_inrepo_dest(lockfile_pkg, lockfile_filename):
+        return os.path.join(
+            "src",
+            "python",
+            lockfile_pkg.replace(".", os.path.sep),
+            lockfile_filename,
+        )
+
+    def lockfile_buildroot_filename(lockfile_name):
+        return os.path.join(buildroot, lockfile_name)
+
     pants_repo_root = get_buildroot()
     touch(os.path.join(buildroot, "pants.toml"))
     backends = sorted({tool.backend for tool in tools})
@@ -287,19 +298,24 @@ def generate(buildroot: str, tools: Sequence[Tool], args: Sequence[str], dry_run
         logger.info("Would run: " + " ".join(args))
         return
 
+    # If there is a pre-existing lockfile, seed it so we get the pretty lockfile diff
+    for tool in tools:
+        lockfile_pkg, lockfile_filename = tool.cls.default_lockfile_resource
+        lockfile_dest = lockfile_inrepo_dest(lockfile_pkg, lockfile_filename)
+        if os.path.isfile(lockfile_dest):
+            logger.debug(f"copying existing lockfile from {lockfile_dest}")
+            shutil.copy(lockfile_dest, lockfile_buildroot_filename(tool.lockfile_name))
+
     logger.debug("Running: " + " ".join(args))
     subprocess.run(args, cwd=buildroot, check=True)
 
     # Copy the generated lockfiles from the tmp repo to the Pants repo.
     for tool in tools:
         lockfile_pkg, lockfile_filename = tool.cls.default_lockfile_resource
-        lockfile_dest = os.path.join(
-            "src",
-            "python",
-            lockfile_pkg.replace(".", os.path.sep),
-            lockfile_filename,
+        shutil.copy(
+            lockfile_buildroot_filename(tool.lockfile_name),
+            lockfile_inrepo_dest(lockfile_pkg, lockfile_filename),
         )
-        shutil.copy(os.path.join(buildroot, tool.lockfile_name), lockfile_dest)
 
 
 def main() -> None: