Fix CmdAction env var test under Windows
Fix CmdAction env var test under Windows

The "TestCmdAction.test_env" test contained a problem: the current
environment should be explicitly copied, and amended with the extra
environment variables to set, instead of passing in an otherwise empty
environment. This caused the tests to (rightly) fail on Windows CI
hosts.

pydoit/doit,saimn/doit,JohannesBuchner/doit