Add Tag to Fully Skip Builds

[KetanReddy]

For changes like #480 it seems extraneous to run the full pipeline for build and test. Even though every stage should hit the Bazel cache it still leads to a longer than zero turn around time to ship changes like this.

• Auto should support this tag
• The first step in the pipeline should be to check the tag and see if we can skip the build. If so we should report success.

[KetanReddy]

Tested out with #498 but it looks like circleci-agent step halt only stops the job and not the pipeline. We'd probably need to do something like this which is a bit more complex and could allow that secret to get leaked in forked builds.