Functions on App Platform: `npm: Command not found` during remote build

I have a node.js function set up, which works fine using the CLI to deploy to a development workspace. Now I’m trying to deploy it as part of an app. I push the code to GitHub and the build is triggered, but I get this:

[2023-03-09 16:39:12] ╭──────────── git repo clone ───────────╼
[2023-03-09 16:39:12] │  › fetching app source code
[2023-03-09 16:39:12] │ => Selecting branch "deploy"
[2023-03-09 16:39:13] │ => Checking out commit "c670dcbcdada6094f6bd002a0e5c7c3f1ab7139a"
[2023-03-09 16:39:13] │ 
[2023-03-09 16:39:13] │  ✔ cloned repo to /.app_platform_workspace
[2023-03-09 16:39:13] ╰────────────────────────────────────────╼
[2023-03-09 16:39:13] 
[2023-03-09 16:39:13]  › applying source directory functions
[2023-03-09 16:39:13]  ✔ using workspace root /.app_platform_workspace/functions
[2023-03-09 16:39:13] 
[2023-03-09 16:39:16]  › validating project.yml configuration
[2023-03-09 16:39:17]  ✔ project.yml is valid
[2023-03-09 16:39:17]  › preparing build environment
[2023-03-09 16:39:18] 
[2023-03-09 16:39:18] ╭──────────── project build ───────────╼
[2023-03-09 16:39:19] │ Started running npm install && npm run build in /.app_platform_workspace/functions/lib
[2023-03-09 16:39:19] │ /bin/bash: line 1: npm: command not found
[2023-03-09 16:39:19] │ 
[2023-03-09 16:39:19] │  ›   Error: 'npm install' exited with code 127
[2023-03-09 16:39:19] │ 
[2023-03-09 16:39:19] │ command exited with code 1
[2023-03-09 16:39:19] │ 
[2023-03-09 16:39:20] │  ✘ build failed

The remote build process seems unable to find its own npm binary. Any advice will be gratefully received.