Add support for pnpm

[ofhouse]

pnpm in tf-next build command is currently not supported.

ToDo

• Update @vercel/build-utils
  Build utils already has support for recognising pnpm: https://github.com/vercel/vercel/blob/a400b9b29d171232f4649e67e1fdcf4fb8d6a4c8/packages/build-utils/src/fs/run-user-scripts.ts#L293
• Add e2e test for pnpm & pnpm monorepo

[RomainBillot]

Hi, can we get help on how to contribute to this one?
I'd really like to see this feature to be in 🚀

[ofhouse]

Hey, since the @vercel/build-utils was already bumped to the correct version in the last release, only@vercel/nft needed an update to make it work.
No time for extensive testing, but should do the trick.
Will release an update soon.

[ofhouse]

This is now released in v0.13.1.