mb: add config for linux-rel-swarming-staging

This is to fix
https://luci-milo-dev.appspot.com/p/chromium/builders/ci/linux-rel-swarming-staging/1795

Bug: b/144318226
Change-Id: I5a28290b1791d8140932ae224faee8973ce59149
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106706
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@google.com>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
Cr-Commit-Position: refs/heads/master@{#750908}

tools/mb/mb_config.pyl

@@ -524,8 +524,9 @@
       # This should be the same with 'android-marshmallow-arm64-rel'.
       'android-marshmallow-arm64-rel-swarming':  'gpu_tests_android_release_bot_minimal_symbols_arm64_fastbuild',
 
-      # This should be the same with 'Linux Builder'.
+      # These should be the same with 'Linux Builder'.
       'linux-rel-swarming': 'gpu_tests_release_bot',
+      'linux-rel-swarming-staging': 'gpu_tests_release_bot',
 
       # This should be the same with 'Mac Builder'.
       'mac-rel-swarming': 'gpu_tests_release_bot_minimal_symbols',