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- [x] Add new / update outdated documentation

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[project]
name = "pkg_test_graph3"
version = "1.0.0"
authors = []

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[project]
name = "pkg_test_graph3"
version = "1.0.0"
authors = []
