nixos/cupsd: use cups-files.conf
cups-filters: Fix banners

It was looking in cups' prefix for the PDF templates. So for instance
printing a CUPS test page wouldn't work.
cups-filters: Fix finding GS in several filters
cups-filters: Fix gstoraster

It was trying to find "gs" via execve, so use execvpe instead. It's
probably better to use gs's absolute path, but maybe not every
cups-filters user needs it.
cupsd.nix: Clean up environment.etc
cups: Get rid of obsolete banners

These use bannertops, which no longer exists. And they conflicted with
the working banners in cups-filters.
cups: Build with SERVERROOT set to /etc/cups