ci: fix pullreq number may not get correctly (#12127)

* Revert "remove save-pr-number"

This reverts commit 085f4bd769.

* Revert "Update report-api-diff.yml"

This reverts commit b73daf4c0e.

* Revert "Update report-api-diff.yml"

This reverts commit cbf2b5ad8a.

* Revert "try to get pull request id from github.event.workflow_run.pull_requests"

This reverts commit 07517ce501.

.github/workflows/get-api-diff.yml

@@ -171,3 +171,16 @@ jobs:
         path: api-head.json
     - name: Kill Misskey Job
       run: screen -S misskey -X quit
+
+  save-pr-number:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Save PR number
+        env:
+          PR_NUMBER: ${{ github.event.number }}
+        run: |
+          echo "$PR_NUMBER" > ./pr_number
+      - uses: actions/upload-artifact@v3
+        with:
+          name: api-artifact
+          path: pr_number