workflow: push CNAME to resulting site

Due to how our github-pages-deploy-action container works, without
adding the CNAME from the main branch, it will be deleted. This will
keep it there.

Note that this should be reverted and CNAME removed, should the site
remain on the default github.io domain.

.github/workflows/publish.yml

@@ -45,6 +45,7 @@ jobs:
         # miscellaneous files needed for GitHub etc
         run: |
           touch site/.nojekyll
+          cp CNAME site/CNAME
 
       - name: Deploy 🚀
         uses: JamesIves/[email protected]