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- [x] Add new / update outdated documentation

package:
  name: pkg_1
  version: 1.0.0
source:
  sha256: deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef
  url: https://dummy-url-that-not-exists.com/pkg_1.tar.gz
requirements:
  host:
    - pkg_1_1
    - pkg_3
    - libtest_shared
  run:
    - pkg_1_1
    - pkg_3
    - libtest_shared
test:
  imports:
    - pkg_1

package:
  name: pkg_1
  version: 1.0.0
source:
  sha256: deadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeefdeadbeef
  url: https://dummy-url-that-not-exists.com/pkg_1.tar.gz
requirements:
  host:
    - pkg_1_1
    - pkg_3
    - libtest_shared
  run:
    - pkg_1_1
    - pkg_3
    - libtest_shared
test:
  imports:
    - pkg_1
