build: agda-build-system: Do not rely on "locales" being present.

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

guix/build-system/agda.scm

@@ -69,7 +69,6 @@
                                 (list "ghc" (default-haskell))
                                 (standard-packages))
                                '())
-                         ,(assoc "locales" (standard-packages))
                          ,@native-inputs))
          (outputs outputs)
          (build agda-build)

guix/build/agda-build-system.scm

@@ -29,7 +29,8 @@
 
 (define* (set-locpath #:key inputs native-inputs #:allow-other-keys)
   (let ((locales (assoc-ref (or native-inputs inputs) "locales")))
-    (setenv "GUIX_LOCPATH" (string-append locales "/lib/locale"))))
+    (when locales
+      (setenv "GUIX_LOCPATH" (string-append locales "/lib/locale")))))
 
 (define %agda-possible-extensions
   (cons