Separate build tests from installation tests

[effigies]

imagecodecs is now required by scikit-image, which has extremely finicky installation requirements, so our old minimum requirements for pip and setuptools are no longer usable. This is mostly because we don't separate the build and install steps when testing.

We can't use old pip/setuptools for python setup.py install, but we should be able to when building wheels and sdists, if we then upgrade pip before installing imagecodecs.

Another option is to set a maximum scikit-image for these old pip/setuptools versions.

This might not be worth it, but I wanted to make a note before the intricacies of how our CI got this way fall down the memory hole.

Follow-up to #520.