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/options.py

@@ -408,6 +408,7 @@ class Options:
             environment_config = self.reader.get(
                 "environment", table={"item": '{k}="{v}"', "sep": " "}
             )
+            environment_pass = self.reader.get("environment-pass", sep=" ").split()
             before_build = self.reader.get("before-build", sep=" && ")
             repair_command = self.reader.get("repair-wheel-command", sep=" && ")
 
@@ -438,6 +439,14 @@ class Options:
                 traceback.print_exc(None, sys.stderr)
                 sys.exit(2)
 
+            # Pass through environment variables
+            if self.platform == "linux":
+                for env_var_name in environment_pass:
+                    try:
+                        environment.add(env_var_name, os.environ[env_var_name])
+                    except KeyError:
+                        pass
+
             if dependency_versions == "pinned":
                 dependency_constraints: Optional[
                     DependencyConstraints