fix: use npm co-located with the action node binary

[benquarmby]

Fixes #234 — spawn npm ENOENT on GHE self-hosted runners where npm is not on PATH.

Root cause

#212 introduced npm ci as the bootstrap mechanism. On standard GitHub-hosted runners npm is on PATH; on GHE self-hosted runners it is not guaranteed to be present.

Fix

• Resolve npm via join(dirname(process.execPath), 'npm'[.cmd]) — npm is always co-located with the node binary that runs the action
• Prepend dirname(process.execPath) to the child PATH so npm's #!/usr/bin/env node shebang can resolve node

Test plan

• GHE self-hosted runner without npm on PATH
• Standard GitHub-hosted Linux, macOS, and Windows runners