Resolve ERROR: invalid tag "ghcr.io/ericstoneking/42:167/merge": in Pull requests

Resolve `ERROR: invalid tag "ghcr.io/ericstoneking/42:167/merge": `in Pull requests when 42's image is being built. This problem only occurs during a pull request.

https://github.com/ericstoneking/42/actions/runs/14939499828/job/41974084977?pr=167#step:6:203