build: add target specific build_type variable

This commit add a target specific variable named 'build_type' to the
node and node_g targets.

The motivation for doing this is that both targets share the same
prerequisites, and the recepies are the same (apart from the build type)
so some duplication can be avoided.

PR-URL: https://github.com/nodejs/node/pull/33925


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>