Allow the use of a `.env` file with the generated program to set environment variables

[syegulalp]

This would be particularly useful to set PYAPP_INSTALL_DIR_<project> for a given project automatically. For instance, I could give the generated executable and a .env file that specifies that the target directory is a subdirectory of the current working directory -- e.g., "./some_dir" - which would make it easier to have control over where the target is unpacked and run from.