Hi,

attached is a simple patch that should print out a warning when a
substitute server is ignored due to a customized store directory.

Motivation: I was checking out guix for the first time a few days ago; I
built it using the --with-store-dir configure option and subsequently,
substitutes were not downloaded. I was baffled for some time because I
thought I did everything according to the manual and yet I couldn't get
the substitute downloads to work (with no indication why).

I suppose the actual reason is quite apparent to people with some
knowledge about guix but for a new user it is not (at least if I may
judge from my own experience). Perhaps this patch will save someone a
little bit of time.

Regards,
Hynek