chore: ensure build is considered production to enable indexing (#698)

Since some time I was wondering why no docs.egi.eu pages are present in
Google search. I just made some verifications and just realised that
until now the indexing by search engines is disabled due the presence of
a meta tag in the source of the pages.

The [docsy theme](https://www.docsy.dev/) version we use relies on
having a `HUGO_ENV` variable set:
https://github.com/google/docsy/blob/f85c845226f30a4b2a55085d142e96def89e4c3f/layouts/partials/head.html#L10-L14

For the record a commit, added after the version we use changes this
behaviour, thus this may have to be revised when updating the theme:
google/docsy@7e2e929

.github/workflows/deploy.yml

@@ -56,6 +56,8 @@ jobs:
 
       - name: Build and minify using Hugo
         run: hugo --minify
+        env:
+          HUGO_ENV: production
 
       - name: Generate token for pushing content
         uses: tibdex/github-app-token@v2