Small improvements in the generation of the PR that updates the protobuf files (envoyproxy#206)

Signed-off-by: rulex123 <[email protected]>

Author: rulex123

.github/workflows/update-protobuf.yml

@@ -21,8 +21,13 @@ jobs:
         uses: peter-evans/create-pull-request@v3
         with:
           branch: update-protobuf-to-${{ inputs.envoy_version }}
+          base: main
+          author: envoy-bot <[email protected]>
+          committer: envoy-bot <[email protected]>
           signoff: true
           title: '[protobuf] Update protobuf definitions to ${{ inputs.envoy_version }}'
+          commit-message: |
+            [protobuf] Update protobuf definitions to ${{ inputs.envoy_version }}
           body: |
             This is an automatic PR created by github action workflow:
             - Updated protobuf files