bug#28310: guix build -n misses package builds

[Andreas Enge]

Hello,

I am right now in the process of updating pari-gp to version 2.9.3.
After building it on a git check-out of three days ago, which went smoothly,
I rebased my patch on today's master and was pleased to see that no rebuild
was needed:

$ ./pre-inst-env guix build pari-gp -n
outputs nothing.

However, once the -n dropped, the gd package gets built.
And then it is starting ruby, which has no connection to pari-gp:
Downloading 
https://mirror.hydra.gnu.org/guix/nar/229n3pzp5bdmbdvwslg0dxliysas92k5-ruby-2.4.1.tar.xz...
   ruby-2.4.1.tar.xz  9.5MiB                                                    
                                                   22KiB/s 00:06 [              
      ]   1.3%^

Andreas