BLD Add DISTDIR variable. ### Description

Adds `DISTDIR` env variable that points to the directory where libraries should be copied to when building shared libraries with recipe.

### Checklists

- [x] Add new / update outdated documentation

emsdk/emsdk/
cpython/installs/
cpython/build/
packages/*/build/

emsdk/emsdk/
cpython/installs/
cpython/build/
packages/*/build/
