Use the Debian firmware package instead of the Intel-specific ones

The intel-*.nix modules are obsolete (since you can just say
‘hardware.enableAllFirmware = true’, or equivalently,
‘hardware.firmware = [ pkgs.firmwareLinuxNonfree ]’).  But we'll keep
them around for compatibility.
Update the locate database using a systemd service

This makes it easier to update the database manually ("systemctl start
update-locatedb").

Also, use modern module syntax.
Add /media and /run to the filesystems ignored by updatedb