Refactor CI scripts to better support reproducing bcr_presubmit locally (#2155)

- Default BUILDKITE_ORG to bazel
- Allow overriding repo root for `create_anonymous_repo` and
`prepare_test_module_repo`
- Allow suppressing log for `prepare_test_module_repo`
- Use `--enable_bzlmod` instead of `--experimental_enable_bzlmod` since
Bazel 5 is now dropped.