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>

cibuildwheel/resources/defaults.toml

@@ -7,6 +7,7 @@ archs = ["auto"]
 build-frontend = "pip"
 dependency-versions = "pinned"
 environment = {}
+environment-pass = []
 build-verbosity = ""
 
 before-all = ""