github: switch to checkout v1

V2 handles tags incorrectly
(actions/checkout#290) and the easiest
solution is to simply switch to v1.

.github/workflows/gh-pages.yml

@@ -13,7 +13,7 @@ jobs:
     steps:
 
     - name: Check out repo
-      uses: actions/checkout@v2
+      uses: actions/checkout@v1
       with:
         fetch-depth: 0