Chromium + Fuchsia + Flutter Engine tools use GOMA_DIR to determine the
location of goma's scripts and binaries. Goma installs track some state
separately, and so when a different goma distribution's `compiler_proxy`
is launched and autoninja attempts to run `.cipd_bin/gomacc port`, the
check fails even though goma is running.

Additional context:
https://github.com/flutter/flutter/issues/81510#issuecomment-839489061
Change-Id: I93775628c6efba62bb226ac1995301c3ae7f5fa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2891250Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Brandon DeRosier <bdero@google.com>