bug#51801: No warning when attempting to build unsupported system with --system

[Maxim Cournoyer]

Hello Guix,

I was surprised that:

--8<---------------cut here---------------start------------->8---
guix build fpc --system=armhf-linux
--8<---------------cut here---------------end--------------->8---

would send me in a dead end without any word of caution, given that
armhf-linux is *not* listed in the supported-systems field.

We should at least warn near the beginning of the build, and remind of
that fact at the end of the build in case of failure.

Thanks,

Maxim