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- [x] Add new / update outdated documentation

# API Reference

```{eval-rst}
.. toctree::
   :maxdepth: 2

   api/js-api.md
   api/python-api.md
   Micropip API <https://micropip.pyodide.org/en/stable/project/api.html>
   api/pyodide-cli.md
   api/pyodide-build-cli.md
```

# API Reference

```{eval-rst}
.. toctree::
   :maxdepth: 2

   api/js-api.md
   api/python-api.md
   Micropip API <https://micropip.pyodide.org/en/stable/project/api.html>
   api/pyodide-cli.md
   api/pyodide-build-cli.md
```
