Corepack not installed with NodeJS 25 by default

[sureshjoshi]

nodejs/nodejs.org#7555

This doesn't affect us yet, since we'll be on LTS generally, but if someone setups NodeJS 25, it could cause this to fall over (but, it might use the Pants corepack instead - I honestly have no idea).

This will become a bigger issue when we hit NodeJS 26 LTS though.

Need to install via userland, if we want to keep using this mechanism:
https://nodejs.org/docs/v24.11.0/api/corepack.html#corepack
https://github.com/nodejs/corepack

Part of the convenience of corepack is not just installing the default package managers, but also being able to pick up and run the package manager in the package.json file.

I think we'll now need to maintain a separate corepack subsystem to keep functionality.