unix/main: Use OS-dependent path separator when searching path.

ports/unix/main.c

@@ -493,7 +493,7 @@ MP_NOINLINE int main_(int argc, char **argv) {
         #endif
     }
     size_t path_num = 1; // [0] is for current dir (or base dir of the script)
-    if (*path == ':') {
+    if (*path == PATHLIST_SEP_CHAR) {
         path_num++;
     }
     for (char *p = path; p != NULL; p = strchr(p, PATHLIST_SEP_CHAR)) {