ci: pin hugo version to 0.139.4 due to special 0.139.5 release

Release 0.139.5 contains no assets and was only made for technical
reasons. The deployment fails with error:

    Error: Action failed with error Unexpected HTTP response: 404

See gohugoio/hugo#13150

.github/workflows/gh-pages.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Setup Hugo
         uses: peaceiris/actions-hugo@v3
         with:
-          hugo-version: 'latest'
+          hugo-version: '0.139.4'
           extended: true
 
       - name: Build