This change adds basic scaffolding to allow configuring hashed_mirrors that will be used
by fetchurl to download artifacts by their hash, this is useful in case certain URLs are
no longer available but required to bootstrap nixpkgs stdenv.
These urls will have higher priority than the url specified in fetchurl(and friends) and
will be attempted before falling back to the actual url specified in fetchurl.
Change-Id: I589bdef609075f274cbdf6b26af602cafaa7496a
Reviewed-on: https://cl.snix.dev/c/snix/+/30567
Tested-by: besadii
Reviewed-by: Florian Klink <flokli@flokli.de>