The current system python in the docker baseimage is py3.9, while the
targetpython and hostpython built by p4a are py3.8, and this was causing
linker issues in the pyqt5 build.
It is also cleaner IMHO to have p4a handle what is needed for a recipe
instead of assuming it is available in the system. (and I think this is
how other existing recipes work)
fixes https://github.com/spesmilo/electrum/issues/8016
