set periodic workflow

.github/workflows/check_nightly.yml

@@ -0,0 +1,26 @@
+name: run build on current nightly
+
+on:
+  schedule:
+    - cron: '0 14 1 * *'
+  workflow_dispatch:
+
+jobs:
+  build:
+    name: build periodic nightly
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout repo
+        uses: actions/checkout@v4
+        with:
+          ref: nightly
+      - name: install elan
+        run: |
+          set -o pipefail
+          curl https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh -sSf | sh -s -- --default-toolchain none -y
+          ~/.elan/bin/lean --version
+          echo "$HOME/.elan/bin" >> $GITHUB_PATH
+      - name: override elan default
+        run: elan override set leanprover/lean4:nightly
+      - name: build
+        run: lake build