fix: attempt to build by replacing npm with pnpm

[codeTrackLift]

@HudsonGraeme

This is my first time using pnpm, but at a glance, it looks like it might not be compatible with our gh-pages build-deploy workflow?

I also tried modifying build-deploy.yml to run pnpm instead of npm, but it was still looking for a package-lock.json (see error message below).