[Bug?]: `nodeExperimentalPackageMap` + corepack

[mrginglymus]

Self-service

• I'd be willing to implement a fix

Describe the bug

Executing a yarn script, where yarn is managed by corepack which has been installed into the global node_modules folder results in a fatal error:

Error [ERR_PACKAGE_MAP_EXTERNAL_FILE]: Cannot resolve "corepack/package.json" from "/home/ubuntu/actions-runner/_work/_tool/node/26.4.0/x64/lib/node_modules/corepack/dist/lib/corepack.cjs": file is not within any package defined in /home/ubuntu/actions-runner/_work/[....]/node_modules/.package-map.json

To reproduce

Using node 26 (supporting package maps),

nodeLinker: pnpm
nodeExperimentalPackageMap: true

npm install -g corepack
corepack enable
yarn tsc --noEmit

Environment

System:
    OS: Windows 11 10.0.26200
    CPU: (24) x64 Intel(R) Core(TM) i9-10920X CPU @ 3.50GHz
  Binaries:
    Node: 26.4.0 - C:\Users\[...]\AppData\Local\Temp\xfs-4c02d330\node.CMD

Additional context

Fails on Windows + Linux in the same way. Appears only to occur with pnpm node linker.