Railpack - canary release of Bun

Question

Is it possible to specify a canary release of Bun to be used when building with Railpack? I tried:

`RAILPACK_BUN_VERSION=1.4.0`

and

```json
  "engines": {
    "bun": ">=1.4.0"
  }
}
```

but image still builds with this logged:

```
Packages  
  ──────────
  bun   │  1.3.14   │  package.json > engines > bun (>=1.4.0)
  node  │  22.23.1  │  railpack default (22)
```

Was hoping to play around with the Rust-rewrite of Bun (v1.4) which is still canary.