basic/env-util: correctly parse extended vars after non-extended vars (#20941)

src/basic/env-util.c

@@ -615,6 +615,7 @@ char *replace_env_n(const char *format, size_t n, char **env, unsigned flags) {
 
                                 word = e+1;
                                 state = WORD;
+                                nest--;
                         } else if (*e == ':') {
                                 if (flags & REPLACE_ENV_ALLOW_EXTENDED) {
                                         len = e - word - 2;