Error: Cannot find module './pnpm.cjs' when trying to build an Nx monorepo app

I might just be off the beaten track here with pnpm, as this looks like a build system error rather than anything to do with my codebase. Basically I have a fairly standard Nx monorepo, pnpm as the package manager, and have NIXPACKS_NX_APP_NAME=api in the build pipeline. This should be enough to get Nixpacks building according to https://nixpacks.com/docs/providers/node But the build fails with the below error:

Node | Nixpacks

App source + Nix packages + Docker = Image