fix sql benchmark workflow permissions (#2367)

I overrode the caller's permissions in the new workflow, and copy-pasted
the permissions from the *-pr workflow which obviously wouldn't work for
the post-merge one.

.github/workflows/sql-benchmarks.yml

@@ -7,12 +7,6 @@ on:
         required: true
         type: string
 
-permissions:
-  actions: write
-  contents: read
-  pull-requests: write
-  id-token: write
-
 jobs:
   tpch:
     runs-on: [self-hosted, gcp]