add checkout step

.github/workflows/publish.yaml

@@ -0,0 +1,21 @@
+name: Publish
+
+on:
+  push:
+    tags:
+      - '*'
+
+jobs:
+  build:
+    name: Publish CPM.cmake
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Upload CPM.cmake to release
+        uses: svenstaro/upload-release-action@v1-release
+        with:
+          repo_token: ${{ secrets.GITHUB_TOKEN }}
+          file: cmake/CPM.cmake
+          asset_name: CPM.cmake
+          tag: ${{ github.ref }}
+          overwrite: true