A "locales" package is no longer included in the 'standard-packages'
collection, per commit de9d6d12 (gnu: commencement: Remove
‘glibc-utf8-locales’ from ‘%final-inputs’."), which is made because glibc now
embeds a C.UTF8 locale.
* guix/build-system/agda.scm (lower): Remove "locales" implicit input.
* guix/build/agda-build-system.scm (set-locpath): Do not fail when a locales
package is not available.
Change-Id: If14e074e457c625960d70fcaf46c7e829d5d58d1