Document the download path `COREPACK_HOME` env and its default path `.node/corepack`

[xnuk]

Related:

corepack/sources/folderUtils.ts

Lines 5 to 7 in 1393e44

	export function getInstallFolder() {
	return process.env.COREPACK_HOME ?? join(homedir(), `.node/corepack`);
	}

[DaAwesomeP]

This really should be documented. My applications run as system users without a home folder. This env var allows that to work.