nixos/gpsd-service: change from deprecated 'jobs' type to 'systemd'

This has the nice side-effect of making gpsd actually run!

Old behaviour (debugLevel=2):

  systemd[1]: gpsd.service holdoff time over, scheduling restart.
  systemd[1]: Stopping GPSD daemon...
  systemd[1]: Starting GPSD daemon...
  systemd[1]: gpsd.service start request repeated too quickly, refusing to start.
  systemd[1]: Failed to start GPSD daemon.
  systemd[1]: Unit gpsd.service entered failed state.

New behaviour (debugLevel=2):

  gpsd[945]: gpsd: launching (Version 2.95)
  systemd[1]: Started GPSD daemon.
  gpsd[945]: gpsd: listening on port 2947
  gpsd[945]: gpsd: running with effective group ID 27
  gpsd[945]: gpsd: running with effective user ID 23
  gpsd[945]: gpsd: stashing device /dev/ttyUSB0 at slot 0
gpsd: update 2.95 -> 3.10

Build system is changed, gpsd uses scons now.

The expressions needs some more work, TODO comments added. (xgps didn't
work before this change either.)