guix build: Add '--check'.

* guix/derivations.scm (build-derivations): Add optional 'mode'
parameter.
* guix/scripts/build.scm (%default-options): Add 'build-mode'.
(show-help, %options): Add '--check'.
(guix-build): Honor 'build-mode' key of OPTS.  Pass it to
'show-what-to-build' and 'build-derivations'.
* doc/guix.texi (Invoking guix build): Document it.
(Substitutes): Mention it.