Bug 1712382: `./mach clobber gradle` works if directory already gone r=nalexander

Back-to-back `./mach clobber gradle` invocations were failing
because `shutil.rmtree(..)` would complain that the target directory
didn't exist.

Differential Revision: https://phabricator.services.mozilla.com/D116129

python/mozbuild/mozbuild/mach_commands.py

@@ -329,7 +329,7 @@ class Clobber(MachCommandBase):
             )
 
         if "gradle" in what:
-            shutil.rmtree(mozpath.join(self.topobjdir, "gradle"))
+            shutil.rmtree(mozpath.join(self.topobjdir, "gradle"), ignore_errors=True)
 
         return ret