[8.5.0] Don't use the gRPC downloader for `file:` URLs

[bazel-io]

Forked from #27799

[iancha1992]

@fmeum I've added the soft-release-blocker. Please let me know if you want it removed. Thank you!

[bazel-io]

Cherry-pick was attempted but there were merge conflicts in the following file(s). Please resolve manually.

src/main/java/com/google/devtools/build/lib/remote/downloader/GrpcRemoteDownloader.java src/test/java/com/google/devtools/build/lib/remote/downloader/GrpcRemoteDownloaderTest.java

cc: @bazelbuild/triage

[iancha1992]

Cherry-picked in https://github.com/bazelbuild/bazel/pull/28026