Updating crazy-max/ghaction-github-page to v2

Deploying is failing with the error

`Error: EEXIST: file already exists, symlink ...`

According to
crazy-max/ghaction-github-pages#121
the issue is fixed in newer versions. Attempting to use v2 to see if it
fixes the issue.

.github/workflows/deploy.yml

@@ -33,7 +33,7 @@ jobs:
           MIX_ENV: prod
       - name: Deploy
         if: success()
-        uses: crazy-max/ghaction-github-pages@v1
+        uses: crazy-max/ghaction-github-pages@v2
         with:
           target_branch: production
           build_dir: _build/prod