nixos/printing: fix CUPS `SetEnv` directive placement

With CUPS v2.3b5, the configuration directive `SetEnv`
moved from `cupsd.conf` to `cups-files.conf`.  See also
https://github.com/apple/cups/commit/d47f6aec436e0e9df6554436e391471097686ecc .

We have to follow up as `SetEnv` is now ignored in `cupsd.conf`.
Without this, executables called by cups
can't find other executables they depend on,
like `gs` or `perl`.