test: add `rules_git` tests

To effectively test `bazel-git` with `rules_git` a repository context
time label to a built `bazel-git` needs to be provided.

The `test/build` repository provides a repository context time build
of the project to retrieve a `bazel-git` that can be used in the
`git_fetch` rules in `MODULE.bazel`.

The downside is that an isolated output base must be used to prevent
deadlock when running the build. Duplicated downloads of dependencies
will occur.

The upside is that `bazel-git` is tested in the same way that the
primary downstream ruleset uses the binary.