Buildtools release artifacts are unsigned and have no checksum files

[wade-arista]

For the release binaries in github.com/bazelbuild/bazel, detached signatures are already provided. The release server also has a signature for the source archive, e.g.

https://releases.bazel.build/7.0.2/release/bazel_7.0.2.tar.gz https://releases.bazel.build/7.0.2/release/bazel_7.0.2.tar.gz.sig

It would certainly be nice to have the same for buildtools, if possible.