Inject environment variables for subprocess

[morganics]

Is your feature request related to a problem? Please describe.

Using os.environ can cause issues in azure_cli.py, e.g. cannot find the az command even when az is on the path.

Describe the solution you'd like
Inject os environment variables in to the identity module, so that we're not dependent on the global os.environ variables.

e.g. line 223 on azure_cli.py

        working_directory = get_safe_working_dir()

        kwargs: Dict[str, Any] = {
            "stderr": subprocess.PIPE,
            "stdin": subprocess.DEVNULL,
            "cwd": working_directory,
            "universal_newlines": True,
            "timeout": timeout,
            "env": dict(self._environ, AZURE_CORE_NO_COLOR="true"), # rather than using os.environ.
        }
        return subprocess.check_output(args, **kwargs)

Describe alternatives you've considered
Patching os.environ, which isn't great.

Additional context
Messy PATHs can stop the az command being found, despite shutil locating the appropriate path.

[xiangyan99]

Thanks for the feedback, we’ll investigate asap.