`networking.firewall.checkReversePath` was being set to "loose" from
Mullvad VPN, which was causing an issue with the kernel used by the
PinePhone with Mobile NixOS.
By changing this option to `false`, we get rid of the "This kernel does
not support rpfilter" error, which seems to be inaccurate due to the
result of `sysctl -a | grep \\.rp_filter` on the phone being consistent
with the result on the laptop.
This fixes an issue where previously the derivation had to be evaluated
before importing the base16 scheme, thus causing `nix flake check` to
fail when multi-platform support was added.
See: https://github.com/NixOS/nix/issues/4265
