chore: add nightly testing YAML to test with

.github/workflows/nightly-test.yml

@@ -0,0 +1,39 @@
+name: Bump lean-toolchain on nightly-testing
+
+on:
+  workflow_dispatch:
+  # schedule:
+  #   - cron: '0 10/3 * * *'  # Run every three hours, starting at 11AM CET/2AM PT.
+
+jobs:
+  update-toolchain:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v4
+      with:
+        ref: nightly-testing # checkout nightly-testing branch
+        token: ${{ secrets.NIGHTLY_TESTING }}
+
+    - name: Get latest release tag from leanprover/lean4-nightly
+      id: get-latest-release
+      run: |
+        RELEASE_TAG="$(curl -s "https://api.github.com/repos/leanprover/lean4-nightly/releases" | jq -r '.[0].tag_name')"
+        echo "RELEASE_TAG=$RELEASE_TAG" >> "${GITHUB_ENV}"
+
+    - name: Update lean-toolchain file
+      run: |
+        echo "leanprover/lean4:${RELEASE_TAG}" > lean-toolchain
+
+    - name: Commit and push changes
+      run: |
+        git config user.name "verso-lean-bot"
+        git config user.email "[email protected]"
+        git add lean-toolchain
+        # Don't fail if there's nothing to commit
+        git commit -m "chore: bump to ${RELEASE_TAG}" || true
+        # rebase on top of main - this branch should only contain toolchain updates
+        git fetch origin/main
+        git rebase origin/main
+        git push -f origin nightly-testing