Bug 1575288 - Shutdown sccache from the right place after bug 1573435. r=nalexander

Differential Revision: https://phabricator.services.mozilla.com/D42762

--HG--
extra : moz-landing-system : lando

testing/mozharness/mozharness/mozilla/building/buildbase.py

@@ -1624,7 +1624,8 @@ or run without that action (ie: --no-{action})"
         '''If sccache was in use for this build, shut down the sccache server.'''
         if os.environ.get('USE_SCCACHE') == '1':
             topsrcdir = self.query_abs_dirs()['abs_src_dir']
-            sccache = os.path.join(topsrcdir, 'sccache', 'sccache')
+            sccache_base = os.environ['MOZ_FETCHES_DIR']
+            sccache = os.path.join(sccache_base, 'sccache', 'sccache')
             if self._is_windows():
                 sccache += '.exe'
             self.run_command([sccache, '--stop-server'], cwd=topsrcdir)