test: skip pack checks while a build holds dist

The pack integrity tests failed whenever another run was writing
dist/, reporting a corrupt archive when the only fact established was
that somebody else was building. Which test went red depended on how
far along that build was. They skip now, the way validate_schemas
already does.

tests/test_pack_integrity.py

@@ -74,6 +74,12 @@ class PackIntegrityTest(unittest.TestCase):
         )
         if result.returncode == 0:
             return
+        # A build holding the exclusive lock means the packs on disk are
+        # half-written. Failing then reports a corrupt archive when the only
+        # fact established is that somebody else is building, and which test
+        # goes red depends on how far along that build is.
+        if "is in use by another run" in (result.stdout + result.stderr):
+            self.skipTest(f"dist/ is being written; {platform_name} not verifiable")
         newer = _profiles_newer_than(pack)
         if newer:
             self.skipTest(