Bug 1499196 - Use outputs as the key for rust commands in Tup once again. r=mshal

Now that we're outputing a stub file for each build script we can index these
commands by their output to de-duplicate them in the tup backend.

Differential Revision: https://phabricator.services.mozilla.com/D9042

python/mozbuild/mozbuild/backend/tup.py

@@ -966,7 +966,7 @@ class TupBackend(CommonBackend):
             invocation['full-deps'].update(inputs)
             invocation['full-deps'].update(invocation['outputs'])
 
-            cmd_key = ' '.join(command)
+            cmd_key = ' '.join(outputs)
             if cmd_key not in self._rust_cmds:
                 self._rust_cmds.add(cmd_key)
                 # We have some custom build scripts that depend on python code