The testsuite driver requires python version to be at least 3.7, but
distro-provided python3 in certain images are too old. This commit
adds logic to install latest python3 in those images. The installed
python3 does not interfere with systemwide python3 and does not appear
in PATH. The PYTHON environment variable will be picked up by the ghc
configure script and be used by hadrian to run the driver.