check-releases workflow should not fail if `beta` or `experimental` branches are missing from DF

[ab9rf]

Tarn recently deleted the beta and experimental branches from the Steam DF repository, which was causing check-releases to fail. I have commented those out from the workflow definition for now. Ideally the workflow should check them if they exist but not fail if they don't, but I don't have the bandwidth right now to make this change.