Plain --print-build-identifiers currently requires --platform

[henryiii]

I was interested to get the identifier I needed for --only, so I ran:

$ pipx run cibuildwheel --print-build-identifiers

cibuildwheel: Unable to detect platform. cibuildwheel should run on your CI server;
Travis CI, AppVeyor, Azure Pipelines, GitHub Actions, CircleCI, Gitlab, and Cirrus CI
are supported. You can run on your development machine or other CI providers
using the --platform argument. Check --help output for more information.

I feel like, especially now with --only, it should instead just print all platform's identifiers, so you know what to pick for --only.

[joerick]

Yeah, that makes sense. For consistency, it should probably print them all even when a platform can be detected. Does that interfere with your GHA build-matrix generation method?