Make GitHub action upload the PDF file

The uploaded PDF file will be available for download for a short time
after the action has completed.

Author: yecril71pl

.github/workflows/check.yml

@@ -25,3 +25,8 @@ jobs:
         run: make quiet
       - name: check-output
         run: ../tools/check-output.sh
+      - name: upload pdf
+        uses: actions/upload-artifact@v2
+        with:
+          name: draft-snapshot
+          path: source/std.pdf