gnu: guix: Embed 'glibc-utf8-locales'.

This will ensure people who download the binary installation tarball
do not get locale-related warnings, provided they use one of the
selected UTF-8 locales.

* gnu/packages/package-management.scm (guix)[arguments]: In
'wrap-program' phase, set GUIX_LOCPATH.
[inputs]: Add GLIBC-UTF8-LOCALES.