Set cargo_manifest_dir if not set

This is to address an `unwrap` in bevy_fmod I intend to remove in a PR.

src/main.rs

@@ -12,6 +12,10 @@ use bevy::asset::ChangeWatcher;
 use crate::prelude::*;
 
 fn main() {
+    if std::env::var_os("CARGO_MANIFEST_DIR").is_none() {
+        std::env::set_var("CARGO_MANIFEST_DIR", ".");
+    }
+
     App::new()
         .add_state::<GameState>()
         .add_systems(Update, state)