action-setup@v6 does not install pnpm version specified in `package_json_file`

This is probably the same underlying cause as #225

When specifying package_json_file and that package.json file has a packageManager field that specifies the version of pnpm to use, the v6 release does not load the specified version.

This results in the following error, which we are currently seeing across tens of repos:

ERR_PNPM_BROKEN_LOCKFILE  The lockfile at "<path>/pnpm-lock.yaml" is broken: expected a single document in the stream, but found more

The packageManager field is set to pnpm 10 in the specified package.json file:

$ grep packageManager package.json
  "packageManager": "pnpm@10.28.1",

The GitHub Action is invoked with the package_json_file input argument only:

    - name: Setup pnpm
      uses: pnpm/action-setup@v6
      with:
        package_json_file: "${{ inputs.working-directory }}/package.json"

Start by reproducing the GitHub Actions invocation with package_json_file pointing to a package.json whose packageManager is pnpm@10.28.1, then inspect how action-setup@v6 resolves that field. Done means the specified pnpm version is installed and the reported broken-lockfile error no longer occurs.