Add support for `.env` files, which can be placed in `$DRIVERS_TOOLS` and in the sub-directories.
The script will first read `$DRIVERS_TOOLS/.env` if it exists, and then `$SCRIPT_DIR/.env` if it exists, to give
the local file higher precedence.

This pattern can be used to replace the use of legacy `${PREPARE_SHELL}`
invocations in a `shell.exec` Evergreen command, enabling the use of `subprocess.exec` instead.