Fix workflow syntax for build-metrix.yml (facebookincubator#9642)

Summary: Pull Request resolved: facebookincubator#9642

Reviewed By: kgpai

Differential Revision: D56669996

Pulled By: kagamiori

fbshipit-source-id: c092b6cafed5290d4414318404e6d742c226539c

.github/workflows/build-metrics.yml

@@ -41,8 +41,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        - runner: "16-core"
-        - type: ["debug", "release"]
+        runner: ["16-core"]
+        type: ["debug", "release"]
     defaults:
       run:
         shell: bash