Suppress SAL logging when running helmfile

Suppress SAL output from helmfile apply when run via scap since it is redundant with scap's own logging. This is performed by setting SUPPRESS_SAL=true in the environment before running helmfile apply. This requires adding an env parameter to subprocess_check_run_quietly_if_ok. This env parameter needs to be initialized to with a copy of os.environ in the helmfile call so we don't erase the existing environment variables.

Bug: T323296