post_push.yml: Use continue-on-error for flaky steps

"Sync git.ruby-lang.org" and "Push PR notes to GitHub" steps can
fail due to transient issues, which blocks execution of all
subsequent steps. Use continue-on-error so the rest of the workflow
still runs, and add a "Check for failures" step at the end to ensure
the overall CI status is still red when either step fails.

example:
https://github.com/ruby/ruby/actions/runs/23825370964/job/69447008480
https://github.com/ruby/ruby/actions/runs/23671724888/job/68966478448

Author: k0kubun

.github/workflows/post_push.yml

@@ -15,6 +15,8 @@ jobs:
     if: ${{ github.repository == 'ruby/ruby' }}
     steps:
       - name: Sync git.ruby-lang.org
+        id: sync-git
+        continue-on-error: true
         run: |
           mkdir -p ~/.ssh
           (umask 066; printenv RUBY_GIT_SYNC_PRIVATE_KEY > ~/.ssh/id_ed25519)
@@ -73,6 +75,8 @@ jobs:
         if: ${{ github.ref == 'refs/heads/master' }}
 
       - name: Push PR notes to GitHub
+        id: pr-notes
+        continue-on-error: true
         run: ruby tool/notes-github-pr.rb "$(pwd)/.git" "$GITHUB_OLD_SHA" "$GITHUB_NEW_SHA" refs/heads/master
         env:
           GITHUB_OLD_SHA: ${{ github.event.before }}
@@ -83,6 +87,10 @@ jobs:
           EMAIL: svn-admin@ruby-lang.org
         if: ${{ github.ref == 'refs/heads/master' }}
 
+      - name: Check for failures
+        run: exit 1
+        if: ${{ steps.sync-git.outcome == 'failure' || steps.pr-notes.outcome == 'failure' }}
+
       - uses: ./.github/actions/slack
         with:
           SLACK_WEBHOOK_URL: ${{ secrets.SIMPLER_ALERTS_URL }} # ruby-lang slack: ruby/simpler-alerts-bot