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- [x] Add new / update outdated documentation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: pydecimal
  version: 1.0.0 # Nonsense
  tag:
    - always
  top-level:
    - _pydecimal
build:
  type: cpython_module
  script: |
    mkdir dist
    export DISTDIR=$(pwd)/dist
    cd $CPYTHONBUILD

    cp Lib/_pydecimal.py $DISTDIR

package:
  name: pydecimal
  version: 1.0.0 # Nonsense
  tag:
    - always
  top-level:
    - _pydecimal
build:
  type: cpython_module
  script: |
    cd $CPYTHONBUILD

    cp Lib/_pydecimal.py $DISTDIR
