make checksum test separate step

Author: mattip

.github/workflows/nikola.yml

@@ -17,9 +17,12 @@ jobs:
     steps:
     - name: Check out
       uses: actions/checkout@v2
-    - name: Build docs
+    - name: Check valid checksums
       shell: bash
       run: python3 check_checksums.py
+    - name: Build docs
+      shell: bash
+      run: |
         sudo apt install virtualenv
         make build
     - name: Deploy