Currently ESP_PYTHON is used in the install.sh script, but it's
overwritten in detect_python.sh. This allows to explicitly specify
python binary, which should be used to create venv.
$ ESP_PYTHON=/usr/bin/python3.7 ./install.{sh,fish}
$ ESP_PYTHON=/usr/bin/python3.7 ./export.{sh,fish}
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>