Build: Set JEKYLL_ENV=production

Regression from v2.1.0 (2750b88), which added support for a dev
mode to search.html based on `jekyll.environment`. However, I
mistakenly thought this defaults to "production" for `jekyll build`
and is only defaulting to "development" for `jekyll serve`.

It actually defaults universally to "development" unless set
differently by the CI build.

Author: Krinkle

.github/workflows/github-pages.yaml

@@ -31,6 +31,8 @@ jobs:
       - name: Jekyll build
         # If your site is in a subdirectory
         # working-directory: ./
+        env:
+          JEKYLL_ENV: production
         run: bundle exec jekyll build
 
       - name: Push to branch