Not much changes, really, other than it now sets PKG_CONFIG_PATH to
point to the python it just installed. This should generally not be a
problem (Meson's python module sets that anyway based on the
executable's introspection data).
Split out checking of file format as a separate GitHub workflow, rather
than running it as part of the project tests for every platform and
toolchain combination in CI, so that this test is not effected by the
changed paths constraints which are applied to the project tests.