[mb] Switch staging bot to gyp for testing purposes

This is to test the migration workflow. Will be reverted
afterwards.

BUG=chromium:616035
NOTRY=true
TBR=vogelheim@chromium.org, jochen@chromium.org

Review-Url: https://codereview.chromium.org/2056673002
Cr-Commit-Position: refs/heads/master@{#36848}

infra/mb/mb_config.pyl

@@ -8,7 +8,7 @@
   # below). MB uses this dict to look up which config to use for a given bot.
   'masters': {
     'client.v8': {
-      'V8 Linux - swarming staging': 'gn_swarming_release',
+      'V8 Linux - swarming staging': 'gyp_swarming_release',
     },
   },