pack: 'self-contained-tarball' runs derivation in a UTF-8local.

* guix/scripts/pack.scm (self-contained-tarball)
[set-utf8-locale]: New variable.
[build]: Use it.
* tests/pack.scm ("self-contained-tarball"): Use a <profile> record
instead of 'profile-derivation'.

tests/pack.scm

@@ -57,10 +57,10 @@
 (unless (network-reachable?) (test-skip 1))
 (test-assertm "self-contained-tarball" %store
   (mlet* %store-monad
-      ((profile (profile-derivation (packages->manifest
-                                     (list %bootstrap-guile))
-                                    #:hooks '()
-                                    #:locales? #f))
+      ((profile -> (profile
+                    (content (packages->manifest (list %bootstrap-guile)))
+                    (hooks '())
+                    (locales? #f)))
        (tarball (self-contained-tarball "pack" profile
                                         #:symlinks '(("/bin/Guile"
                                                       -> "bin/guile"))