Allow to change the location where the files get unpacked

[facundobatista]

This shall be done through the PYEMPAQ_UNPACK_PATH environment variable. The indicated path must exist and be a directory, otherwise it's an error.

It will override the usage of appdirs.user_data_dir() / 'pyempaq'; in other words, the project directory will be created directly in the indicated path.

For example: PYEMPAQ_UNPACK_PATH=/bigdisk/installs python3 superapp.pyz.

[facundobatista]

I'm changing PYEMPAQ_UNPACK_PATH for PYEMPAQ_UNPACK_BASE_PATH to make explicit that it is the base directory (inside it will be created a directory for each project).