If a test needs to, for example, use ssh to connect to another host, it may need to access the ssh agent, which it finds via an environment variable. However, the generated test program sets a minimal, artificial environment at start, which is good, but could do with a way for the user to pass in specific environment variable values. * [ ] Add `--env key=value` support to the `bash` runner * [x] Add `--env key=value` support to the `python` runner * [x] Add `SUBPLOT_ENV_key=value` support to the `rust` codegen