Drop PR body for automated fetches (#1162)

Otherwise, we're always dropping the commit info out of the squashed
commit manually.

Similar to bufbuild/plugins#2278.

Author: stefanvanburen

.github/workflows/fetch.yaml

@@ -48,10 +48,11 @@ jobs:
           branch: fetch-modules
           delete-branch: true
           title: "Found new managed modules references"
-          body: |
-            New managed modules references found. Please review.
+          body: ""
           team-reviewers: bufbuild/core-team
           token: ${{ steps.generate_token.outputs.token }}
+          author: ${{ steps.generate_token.outputs.app-slug }}[bot] <${{ steps.generate_token.outputs.app-slug }}[bot]@users.noreply.github.com>
+          committer: ${{ steps.generate_token.outputs.app-slug }}[bot] <${{ steps.generate_token.outputs.app-slug }}[bot]@users.noreply.github.com>
       - name: Generate Github Token
         id: generate_issues_token
         uses: actions/create-github-app-token@29824e69f54612133e76f7eaac726eef6c875baf # v2.2.1