Allow offline mode via environment variable

[theCapypara]

Hello! I'm trying to publish an app using Flatpak. Flatpak works with completely offline builds where all artifacts are pre-fetched and no internet connection is available in the build environment,

At the moment (as far as I can tell) maturin only supports offline mode via the maturin CLI. This is very tedious to setup in such a build environment, and there seems to be no way to pass this flag using normal pip or other packaging tool installations. As such I'm currently forced to write rather complicated workarounds to install maturin and run it for every module that requires it.

It would be great if offline mode could be enforced using an environment variable, ideally one shared with Setuptools Rust, see this issue:
PyO3/setuptools-rust#363

[messense]

You can use pip install --config-settings build-args='--offline'.

[theCapypara]

Thanks, is this documented anywhere? I went through the documentation and found nothing of the sorts.

[messense]

No, pull requests are welcome to document it.

See also #1619.

[theCapypara]

Thank you. For reference, the following command works in a Flatpak:

. /usr/lib/sdk/rust-stable/enable.sh && cp /app/whl_tmp/tomli-2.0.1-*.whl . && cp /app/whl_tmp/maturin-1.2.3-cp310-cp310-*.whl . && pip3 wheel --verbose --exists-action=i --no-index --find-links=\"file://${PWD}\" --config-settings build-args='--offline' \"./rpds_py\" -w /app/whl_tmp```

It must be noted that Maturin and tomli must be pre-built in a preceding module, because maturin itself currently needs to be patched due to PyO3/setuptools-rust#363