Mk/Uses/npm.mk: Add new variable NPM_PREFETCH_ARCHS

A node modules archive created by the prefetch function may vary
depending on the architecture, especially if the archive contains
architecture-specific node modules.

This variable can be used to indicate which architectures the node
modules archives are prepared for.

If you prepared the archives for amd64 and aarch64 architecutres,
specify:
NPM_PREFETCH_ARCHS= aarch64 amd64