util: treat 'C' and 'POSIX' locale identical

src/basic/util.c

@@ -4273,7 +4273,7 @@ bool is_locale_utf8(void) {
         /* Check result, but ignore the result if C was set
          * explicitly. */
         cached_answer =
-                streq(set, "C") &&
+                STR_IN_SET(set, "C", "POSIX") &&
                 !getenv("LC_ALL") &&
                 !getenv("LC_CTYPE") &&
                 !getenv("LANG");