profiles: Remove dependency on 'glibc-utf8-locales' for tests.

Commit 1af0860e added a mandatory
dependency on 'glibc-utf8-locales', which entails long rebuilds for
tests.

* guix/profiles.scm (profile-derivation): Add #:locales? parameter.
Add 'set-utf8-locale' variable.  Use it when LOCALES? is true.
(link-to-empty-profile): Pass #:locales? #f.
* guix/scripts/environment.scm (inputs->profile-derivation): Pass
  #:locales?.
* guix/scripts/package.scm (build-and-use-profile): Likewise.
* tests/packages.scm ("--search-paths with pattern"): Pass #:locales? #f.
* tests/profiles.scm ("profile-derivation")
("profile-derivation, inputs", "profile-manifest, search-paths")
("etc/profile", "etc/profile when etc/ already exists"):
("etc/profile when etc/ is a symlink"): Likewise.