mkarchiso: set LC_ALL instead of LANG

LC_ALL overrides LANG and all LC_* variables, so use it instead.
See https://www.gnu.org/software/gettext/manual/html_node/Locale-Environment-Variables.html#Locale-Environment-Variables .

archiso/mkarchiso

@@ -6,7 +6,7 @@ set -e -u
 
 # Control the environment
 umask 0022
-export LANG="C"
+export LC_ALL="C"
 export SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-"$(date +%s)"}"
 
 # Set application name from the script's file name