Add a new 'Zola Check' CI workflow

This also checks external links.

We add it as a separate job because it seems to be still buggy.

.github/workflows/build-site.yml

@@ -39,6 +39,20 @@ jobs:
         name: generated_site
         path: blog/public
 
+  zola_check:
+    name: "Zola Check"
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+
+    - name: 'Download Zola'
+      run: curl -sL https://github.com/getzola/zola/releases/download/v0.9.0/zola-v0.9.0-x86_64-unknown-linux-gnu.tar.gz | tar zxv
+
+    - name: "Run zola check"
+      run: ../zola check
+      working-directory: "blog"
+
   check_spelling:
     name: "Check Spelling"
     runs-on: ubuntu-latest