build-gme5: don't try to create worktrees if the submodule was not checked out

Leads to weird states.

build-gem5

@@ -28,6 +28,8 @@ else:
     os.makedirs(binaries_dir, exist_ok=True)
     os.makedirs(disks_dir, exist_ok=True)
     if  not os.path.exists(os.path.join(common.gem5_src_dir, '.git')):
+        if common.gem5_src_dir == common.gem5_default_src_dir:
+            raise Exception('gem5 submodule not checked out')
         subprocess.check_call([
             'git',
             '-C', common.gem5_default_src_dir,