Run benchmark only on main (and rename the f ile (#29142)

.github/workflows/benchmark-prevent-performance-degradations.yml

@@ -7,6 +7,7 @@ on:
 jobs:
   bench:
     name: Bench
+    if: github.base_ref == 'main'
     runs-on: ubuntu-latest
     steps:
       - name: Check out code into the Go module directory