bug#28984: 'guix size' fails with options from GUIX_BUILD_OPTIONS

[Eric Bavier]

The manual describes the GUIX_BUILD_OPTIONS environment variable, that
can affect the operation of guix commands that perform builds.  But
'guix size' can fail when GUIX_BUILD_OPTIONS is set.  E.g.

$ export GUIX_BUILD_OPTIONS='-K --cores=2'
$ guix size dillo
guix size: error: K: unrecognized option

And indeed 'guix size --help' doesn't mention any of the normal
build-type options.

`~Eric