Host index source file in repository

The index source file contains the normalized URL and the metadata that can't be derived from the library releases
("types" data and locked name) for each of the libraries in the Library Manager index. It's most appropriate to store it
in the same repository as the submission list.

.github/workflows/manage-prs.yml

@@ -287,19 +287,20 @@ jobs:
           repo: ${{ github.event.repository.name }}
           pull_number: ${{ github.event.pull_request.number }}${{ github.event.issue.number }}
 
-      - name: Set environment variables
-        run: |
-          echo "INDEX_ENTRY_FILE_PATH=${{ runner.temp }}/index-entry.txt" >> "$GITHUB_ENV"
-
-      - name: Write index entry to file
-        run: |
-          echo "${{ needs.parse.outputs.index-entry }}" >> "${{ env.INDEX_ENTRY_FILE_PATH }}"
-
-      - name: Upload index entry file to workflow artifact
-        uses: actions/upload-artifact@v2
+      - name: Checkout index source branch
+        uses: actions/checkout@v2
         with:
-          path: ${{ env.INDEX_ENTRY_FILE_PATH }}
-          name: index-entry
+          ref: production
+
+      - name: Add index source file entry for submissions
+        run: |
+          INDEX_SOURCE_FILE_PATH="${{ github.workspace }}/repositories.txt"
+          git config --global user.email "bot@arduino.cc"
+          git config --global user.name "ArduinoBot"
+          echo "${{ needs.parse.outputs.index-entry }}" >> "$INDEX_SOURCE_FILE_PATH"
+          git add --update "$INDEX_SOURCE_FILE_PATH"
+          echo -e "Add submission # ${{ github.event.pull_request.number }}${{ github.event.issue.number }}\n\n${{ github.event.repository.html_url }}/pull/${{ github.event.pull_request.number }}${{ github.event.issue.number }}" | git commit --file -
+          git push
 
   request-review:
     needs: