They tend to raise OSErrors.

As pointed out in !299 the libffi shipped with the portable pypy version
conflicts with the system libffi, and could lead to crashes.

Instead of the pyenv version just use the official PPA which contains
a pypy linked against system libs. That has the downside that the version
is no longer pinned and docker image rebuilds will lead to newer pypy versions
being included.

After !301 the pypy version has a different pip version compared to the
oder pyenv cpython installations and also its pip installs to the system
which comes with new restrictions with newer pip (requires --break-system-packages).

To make the test environment for all interepreter versions the same create
and active a venv first and also update pip in there.

This is motivated by the pip in 22.04 not supporting "--config-settings",
which is required in !302

In case meson-python is invoked via pip, any Python tools it uses
can't access the system Python packages, which breaks g-ir-scanner,
which we use for building the tests.

Fix by not building tests when building a wheel.

See https://github.com/mesonbuild/meson-python/issues/596

Fixes #622