chore: upon nightly release, trigger nightly_bump_toolchain on mathlib4 (#4838)

as discussed at
https://leanprover.zulipchat.com/#narrow/stream/428973-nightly-testing/topic/Bumping.20more.20often/near/453976634

Author: nomeata

.github/workflows/ci.yml

@@ -545,3 +545,8 @@ jobs:
           gh workflow -R leanprover/release-index run update-index.yml
         env:
           GITHUB_TOKEN: ${{ secrets.RELEASE_INDEX_TOKEN }}
+      - name: Update toolchain on mathlib4's nightly-testing branch
+        run: |
+          gh workflow -R leanprover-community/mathlib4 run nightly_bump_toolchain.yml
+        env:
+          GITHUB_TOKEN: ${{ secrets.MATHLIB4_BOT }}