This PR updates the OS/BLAS test matrix workflow that is used to test releases against various combinations of operating systems and BLAS implementations. Changes:

• Found and fixed an issue in nyquist_test.py in which some calls with a manually specified frequency response would generate a user warning and others wouldn't. Both cases are not handled properly (and a redundant check has been removed).
• Fixed some issues with paths for OpenBLAS on macos that were causing compiler errors.
• Updated pip-based tests to check out current branch instead of main (mainly useful when using a development branch, like this one).

No changes to the actual control toolbox itself.