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.