CI/CD: use an isolated Guix environment and tag jobs

.gitlab-ci.yml

@@ -2,9 +2,16 @@
 # https://gitlab.com/pages/org-mode.
 
 pages:
+  tags: ["guix"]
   script:
-  - emacs --batch --eval "(progn (package-initialize) (package-refresh-contents) (package-install 'org))"
-  - emacs --batch --no-init-file --load pages.el --funcall org-publish-all
+  # Export the example page to HTML and publish it alongside with the template
+  # sources to the GitLab page.
+  - >
+    guix environment --pure --ad-hoc coreutils which wget emacs emacs-org2web
+    emacs-org emacs-org-contrib emacs-htmlize -- emacs --batch --no-init-file
+    --eval '(setq org-confirm-babel-evaluate nil)' --load pages.el --funcall
+    org-publish-all
+  # Set the example page to be the index page of the GitLab page.
   - mv ./public/example.html ./public/index.html
   artifacts:
     paths: