Fallback to `@TERMUX_PREFIX@/tmp` if env `XDG_RUNTIME_DIR` is not set.

--- a/src/util/anon_file.c
+++ b/src/util/anon_file.c
@@ -122,7 +122,7 @@ get_or_create_user_temp_dir(char* buf, size_t len) {
         return buf;
     }
 
-    snprintf(buf, len, "/tmp/xdg-runtime-mesa-%ld", (long)getuid());
+    snprintf(buf, len, "@TERMUX_PREFIX@/tmp/xdg-runtime-mesa-%ld", (long)getuid());
     mesa_logd("%s: XDG_RUNTIME_DIR not set; falling back to temp dir %s",
         __func__, buf);
     if (stat(buf, &st) == 0) {
