Resolve "Extra tags build overide assume pipeline is for a commit"

Closes #10 (closed)

See the issue for a description of the problem. See the comments in the change for why this isn't an ideal solution - but it does at least mean that the errors go away for now.

For an example of the error before this change see: https://gitlab.developers.cam.ac.uk/uis/devops/tls-certificates/tlscerts/-/jobs/116028. Note that although the build job succeeds, there's actually an error near the end (which is a separate thing we should fix).

For an example using this code see: https://gitlab.developers.cam.ac.uk/uis/devops/tls-certificates/tlscerts/-/jobs/116047. Note that there's no error.