Create a `CIBW_ALLOW_EMPTY` environment variable for `--allow-empty`

[ConorMacBride]

Description

Would it be possible to enable the CLI flag --allow-empty via an environment variable (e.g., CIBW_ALLOW_EMPTY)?

Here's our use case:

• We are running a matrix of build jobs in GitHub Actions.
• One of these jobs is cp313-manylinux_x86_64.
• Nightly builds will set CIBW_PRERELEASE_PYTHONS=1 but otherwise this job will not match any wheels.
• We'd like to set the --allow-empty flag, but it currently doesn't seem possible through the cibuildwheel GitHub Action.
• (More details: FR: support dynamic CIBW_PRERELEASE_PYTHON and CIBW_FREETHREADED_SUPPORT in publish.yml OpenAstronomy/github-actions-workflows#212)

I'm thinking we could compute its value similarly to how CIBW_PRERELEASE_PYTHONS is handled:

cibuildwheel/cibuildwheel/options.py

Lines 523 to 525 in a4d2405

	prerelease_pythons = args.prerelease_pythons or strtobool(
	self.env.get("CIBW_PRERELEASE_PYTHONS", "0")
	)

This could be integrated within options.compute_options and added to options.GlobalOptions:

cibuildwheel/cibuildwheel/options.py

Lines 71 to 77 in a4d2405

	@dataclasses.dataclass(frozen=True)
	class GlobalOptions:
	package_dir: Path
	output_dir: Path
	build_selector: BuildSelector
	test_selector: TestSelector
	architectures: set[Architecture]

Alternatively, CIBW_ALLOW_EMPTY could be checked directly within __main__.py:

cibuildwheel/cibuildwheel/__main__.py

Lines 337 to 338 in a4d2405

	if args.allow_empty:
	print(f"cibuildwheel: {message}", file=sys.stderr)

I'm happy to open a PR if you think a new environment variable would be appropriate for this option. Thanks!

Build log

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CI config

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[joerick]

Sounds good! The first implementation idea above makes the most sense to me.