Update Nikola action version

Author: lfp2

.github/workflows/main.yml

@@ -13,4 +13,4 @@ jobs:
     - name: Check out
       uses: actions/checkout@v2
     - name: Build and Deploy Nikola
-      uses: getnikola/nikola-action@v2
+      uses: getnikola/nikola-action@v8