- it shouldn't be possible to set the SDK version to the same as the new
tag's version. The SDK must always be a previous build.
- don't fail if there isn't any old manifests to git rm.
There are ways to improve, it'd be sexy if it was truly safe and we
could throw around annoying terms like idempotent to make this kind of
sequence just work but doesn't yet: tag_release; tag_release --push
A number of places refer to these paths and that number is going to
grow. Since the standard pattern is to use environment variables for
commonly used paths it is time to add ones for these:
REPO_CACHE_DIR
REPO_MANIFESTS_DIR
