bug#19318: %guix-version is incorrect in the presence of 'guix pull'

[Ludovic Courtès]

Running ‘guix pull’ doesn’t change (@ (guix config) %guix-version),
which gives a wrong impression about the version being (‘guix --version’
will keep showing the same old version number because (guix config) is
never overridden.)

The solution may be to add a (guix version) module, that will itself be
overridden by ‘guix pull’.

(Reported by Tsutsukakushi on #guix.)

Ludo’.