pw_presubmit: Test building Python tree

Add 'build_pigweed_python_source_tree' to presubmits to ensure it
doesn't break.

Change-Id: I16ef6139b6e00f9cb530734f9cca3aafef32dcc1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60500
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
diff --git a/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py b/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
index 3defaf6..3b165a9 100755
--- a/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
+++ b/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
@@ -93,7 +93,8 @@
     build.gn_gen(ctx.root, ctx.output_dir)
     build.ninja(ctx.output_dir, *_at_all_optimization_levels('stm32f429i'),
                 *_at_all_optimization_levels(f'host_{_HOST_COMPILER}'),
-                'python.tests', 'python.lint', 'docs')
+                'python.tests', 'python.lint', 'docs',
+                'pw_env_setup:build_pigweed_python_source_tree')
 
 
 @filter_paths(endswith=_BUILD_EXTENSIONS)