ci/github-script/prepare: fix review dismissals

I broke this in 1c3e149546 (diff-9baa1b84a40210e20622eeb04bea6db28ef3ff90dea79aa54ba9d1e27c150b21).

See #520239 for an example of this not working.

ci/github-script/prepare.js

@@ -181,9 +181,9 @@ module.exports = async ({ github, context, core, dry }) => {
           event: 'REQUEST_CHANGES',
           reviewKey,
         })
+      } else {
+        await dismissReviews({ github, context, core, dry, reviewKey })
       }
-    } else {
-      await dismissReviews({ github, context, core, dry, reviewKey })
     }
 
     let mergedSha, targetSha