guix: don't export TZ twice

contrib/guix/libexec/build.sh

@@ -4,6 +4,9 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 export LC_ALL=C
 set -e -o pipefail
+
+# Environment variables for determinism
+export TAR_OPTIONS="--owner=0 --group=0 --numeric-owner --mtime='@${SOURCE_DATE_EPOCH}' --sort=name"
 export TZ=UTC
 
 # Although Guix _does_ set umask when building its own packages (in our case,
@@ -157,10 +160,6 @@ case "$HOST" in
         ;;
 esac
 
-# Environment variables for determinism
-export TAR_OPTIONS="--owner=0 --group=0 --numeric-owner --mtime='@${SOURCE_DATE_EPOCH}' --sort=name"
-export TZ="UTC"
-
 ####################
 # Depends Building #
 ####################