ci: Fallback to actions/checkout@v2

Apparently there are issues with v3 not being able to checkout with `PUSH_KEY`?

actions/checkout#664

Author: francorbacho

.github/workflows/gh-pages.yml

@@ -18,7 +18,7 @@ jobs:
     - uses: actions/checkout@v3
 
     - name: Checkout pages
-      uses: actions/checkout@v3
+      uses: actions/checkout@v2
       with:
           path: 'pages'
           ref: 'gh-pages'