feat: support passenv (#914)

* feat: support passenv

* refactor: environment-pass and spaces

* docs: document environment pass

* Add unit test for passing through awkward env variable values

* Remove unused as_shell_commands method from environment

* Fix passthrough edge case by avoiding bash for passthrough assignments

* Fix mypy error in test

* Fix unit test

* Apply suggestions from code review

Co-authored-by: Joe Rickerby <joerick@mac.com>

README.md

@@ -112,6 +112,7 @@ Options
 |   | [`CIBW_PRERELEASE_PYTHONS`](https://cibuildwheel.readthedocs.io/en/stable/options/#prerelease-pythons)  | Enable building with pre-release versions of Python if available |
 | **Build customization** | [`CIBW_BUILD_FRONTEND`](https://cibuildwheel.readthedocs.io/en/stable/options/#build-frontend)  | Set the tool to use to build, either "pip" (default for now) or "build" |
 |   | [`CIBW_ENVIRONMENT`](https://cibuildwheel.readthedocs.io/en/stable/options/#environment)  | Set environment variables needed during the build |
+|   | [`CIBW_ENVIRONMENT_PASS_LINUX`](https://cibuildwheel.readthedocs.io/en/stable/options/#environment-pass)  | Set environment variables on the host to pass-through to the container during the build. |
 |   | [`CIBW_BEFORE_ALL`](https://cibuildwheel.readthedocs.io/en/stable/options/#before-all)  | Execute a shell command on the build system before any wheels are built. |
 |   | [`CIBW_BEFORE_BUILD`](https://cibuildwheel.readthedocs.io/en/stable/options/#before-build)  | Execute a shell command preparing each wheel's build |
 |   | [`CIBW_REPAIR_WHEEL_COMMAND`](https://cibuildwheel.readthedocs.io/en/stable/options/#repair-wheel-command)  | Execute a shell command to repair each (non-pure Python) built wheel |