[CI] Fix Cython installation (#4983)

[CI] Fix Cython installation (#4983)

Cython has been causing issues recently, see e.g.
https://github.com/CVC4/CVC4/pull/4982/checks?check_run_id=1052433862.
It looks like the issue is that globally installed packages can't be
found by Python (maybe the global site-package directories changed/are
not included in the search paths anymore?). This commit changes the
installation of Cython to install it locally to the user instead of
globally. It also adds `bin` in the user base directory to `PATH` s.t.
CMake is able to find the `cython` binary.