#239 prepended `dirname(process.execPath)` to PATH so npm's
`#!/usr/bin/env node` shebang resolves on GHE self-hosted runners
where node isn't on PATH. But on runners with a prior `setup-node`
step, this shadowed the user's installed toolchain with the
runner-bundled node (e.g. externals/node24/bin), pairing the user's
npm with a mismatched node — or picking up a broken `npm` shipped
next to the runner-bundled node.
Append instead so a user-installed toolchain on PATH keeps
precedence; the runner's node dir remains as a fallback for the
original #234 case where node isn't on PATH at all.
Fixes#240
