system: Define 'GUIX_LOCPATH' to work around 'glibc' package defect.

Our 'glibc' package currently ignores /run/current-system/locale,
although the intent is to look for locales in that directory.

* gnu/packages/base.scm (glibc/linux): Add comment about
/run/current-system/locale being ignored.
* gnu/system.scm (operating-system-environment-variables): Add
'GUIX_LOCPATH'.
* gnu/tests/base.scm (run-basic-test): Add "locale" test.