blog: add favicon to blog build
Add nix/pkgs/blog.nix to build the blog with proper static assets
including the dolly favicon files. Update the CI workflow to generate
favicons using the dolly tool before building.
The favicon was missing in production because appview/pages/static/* is
gitignored, and the CI workflow wasn't generating the dolly icons.
Signed-off-by: eti <eti@eti.tf>