Fix a boot problem reported by ng0

[cmmarusich]

As you know, ng0 was having trouble booting into GuixSD after
1ef8b72a7f87afe7cffe52393d99e1b14e4770e1.  When I looked closely at the
change, I realized we were not doing the right thing in all cases.  Here is a
patch to fix that.

I've confirmed via manual testing in a QEMU image that when using a device
path for the file system containing the store, 'guix system reconfigure' does
the right thing: it uses a file-based GRUB root search in the new GRUB menu
entry.  That's what it originally did, but
1ef8b72a7f87afe7cffe52393d99e1b14e4770e1 broke that behavior.

I didn't catch this regression because our existing system tests did not catch
it, and in all my manual tests, I was using file system labels, not device
paths.

--
Chris