Disable nix-daemon before upgrading

[kiasoc5]

I don't know if this is the place to post answers to unasked questions, 
but if anyone has the nix service installed and is stuck an error like 
`guix system: error: chown: Read-only file system` when running `guix 
system reconfigure`, restart the nix-daemon service before 
reconfiguring. It took me a while to figure this out from the guix git 
commit log (commit 797be0ea5c3703ad96acd32c98dca5f946cf5c95).