feat(bazel-bot): Support cloning to non-default branches
feat(bazel-bot): Support cloning to non-default branches

The source and target branches are controlled using the
`SOURCE_BRANCH` and `TARGET_BRANCH` environment variables. If a
branch variable isn't set, the default branch for the repo is used
instead. These environment variables are expected to be set in the
trigger running the Docker image.

googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots,googleapis/repo-automation-bots