Add bun_version to link metadata (#25545)

## Summary
- Add `bun_version` field to `link-metadata.json`
- Pass `VERSION` CMake variable to the metadata script as `BUN_VERSION`
env var

This ensures the build version is captured in the link metadata JSON
file, which is useful for tracking which version produced a given build
artifact.

## Test plan
- Build with `bun bd` and verify `link-metadata.json` includes
`bun_version`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

cmake/targets/BuildBun.cmake

@@ -1210,6 +1210,7 @@ if(BUN_LINK_ONLY)
       "Uploading link metadata"
     COMMAND
       ${CMAKE_COMMAND} -E env
+        BUN_VERSION=${VERSION}
         WEBKIT_DOWNLOAD_URL=${WEBKIT_DOWNLOAD_URL}
         WEBKIT_VERSION=${WEBKIT_VERSION}
         ZIG_COMMIT=${ZIG_COMMIT}