HOWTO: cleanup if a release build trigger fails

1. delete the tag it created, e.g. [email protected]
2. delete the branch it created, e.g. auto/version-master-18.0.46
3. re-run the build trigger

Note that comments may be added multiple times to each of the relevant PRs; this is a minor inconvenience...

See also https://github.com/keymanapp/keyman/issues/11640