[Chore] Update development setup

flake.nix: inherit devShells, so that nix develop works
tests: configurable mock server port to avoid conflicts

With these changes I've been able to build the project and run its tests as follows

nix shell nixpkgs#haskellPackages.hpack nixpkgs#cabal-install
nix develop -c $SHELL
hpack
vsftpd \
   -orun_as_launching_user=yes \
   -olisten_port=2221 \
   -olisten=yes \
   -oftp_username=$(whoami) \
   -oanon_root=./ftp-tests/ftp_root \
   -opasv_min_port=2222 \
   -ohide_file='{.*}' \
   -odeny_file='{.*}' \
   -oseccomp_sandbox=no \
   -olog_ftp_protocol=yes \
   -oxferlog_enable=yes \
   -ovsftpd_log_file=./ftp.log &
cabal test ftp-tests --test-options="--ftp-host ftp://127.0.0.1:2221"
cabal test xrefcheck-tests --test-options="--mock-server-port 3001"

serokell / xrefcheck

[Chore] Update development setup #301