bcr_presubmit: Prepare test module source via Bazel (#2382)

- Removed custom logic to download, unpack and patch module sources,
which have subtle differences with how Bazel does it and behaves
differently on different platforms (e.g macOS still had patch 2.0
instead of 2.8)
- Instead, we now use the latest version of Bazel to vendor the module
source.

Tested locally with BCR's `bazel run //tools:setup_presubmit_repos`

Related:
https://github.com/bazelbuild/bazel-central-registry/pull/6056#discussion_r2398025522