pw_env_setup: Don't clear gni vars in CIPD hook

Don't clear the variables in the pigweed_environment.gni file when the
CIPD post-checkout hook runs.

Change-Id: I51ec6fcea4d24cdfca5aaeb975547b6fb5a2e99a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/105157
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
diff --git a/pw_env_setup/py/pw_env_setup/env_setup.py b/pw_env_setup/py/pw_env_setup/env_setup.py
index 6addb8b..23cdbca 100755
--- a/pw_env_setup/py/pw_env_setup/env_setup.py
+++ b/pw_env_setup/py/pw_env_setup/env_setup.py
@@ -378,6 +378,9 @@
             raise MissingSubmodulesError(', '.join(sorted(missing)))
 
     def _write_gni_file(self):
+        if self._cipd_only:
+            return
+
         gni_file = os.path.join(self._project_root, 'build_overrides',
                                 'pigweed_environment.gni')
         if self._gni_file: