remote/correctness: Bazel doesn't track system libraries.

Bazel currently does not track tools outside a workspace. This can be a problem if, for example, an action uses a compiler from `/usr/bin/`. Then, two users with different
compilers installed will wrongly share cache hits because the outputs are different but they have the same action hash.

Start by tracing how Bazel hashes actions and handles remote cache inputs when tools outside the workspace are used, especially compilers such as /usr/bin/. Reproduce the case with two different installed compilers; done means their differing outputs cannot incorrectly share the same remote cache hit.

Written by the indexing model from the issue text.