Add shell environment variables to Dynamic variables feature of platformio.ini

[ohhorob]

A few of my projects share private libraries that each live in their own git repo.

Instead of making each library a git submodule of each project, I'm using the lib_extra_dirs option to have the library dependency finder notice them.

lib_extra_dirs = /Users/rob/arduino/libs

I would like to refer to the user home directory (not platformio home directory) with a dynamic variable in order to avoid absolute references outside of the project.

lib_extra_dirs = ${env.HOME}/arduino/libs

Something like that?