Support using <FORK>/<VERSION> in `.bazelversion` file.

[cgrindel]

It is possible to specify that an integration test uses the Bazel version specified in a .bazelversion file. It would be useful to support Bazelisk's <FORK>/<VERSION>.

[katre]

I am getting errors with a .bazelversion that contains "last_downstream_green", is this the same issue?

[cgrindel]

Yes. Right now, this repo has its own logic for downloading the Bazel binaries. Perhaps, we should rethink this strategy and try to leverage bazelisk to do the binary downloads. 🤔