Add environment variable to pw_cli.process module
Add environment variable to pw_cli.process module

This change sets an environment variable in the subprocess run by
pw_cli.process.run() to indicate that it is running as a subprocess.

Change-Id: I1def3259b62578a156a4b374d41af99692147699

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed