Detect actions that are pinned to master/main

[jmeickle-theaiinstitute]

I updated actionlint to the v1.7.1 which caught a stray actions/checkout@v2 in the codebase, which is awesome.

However, it didn't catch another instance where someone had pinned to actions/checkout@master.

Due to the possibility of pulling in a new major version or even a broken commit, my expectation is that pinning any action to master or main would default to being an actionlint failure. I'd expect this to require an explicit ignore to be accepted.