Only test unset BLAS and MKL on conda package for Windows

[bnavigator]

Hopefully closes actions/toolkit#144

[bnavigator]

Meh: actions/runner#2347

[roryyorke]

This is probably revealing some arcane low-level bug, but I think it's likely not in Slycot; if it is, it's in build configuration.

Shall we drop the Windows Generic builds altogether? conda by default installs MKL, and I guess most people who don't want that use OpenBLAS.

As I understand, we use conda-forge's recommended build method, and so we always compile and link against the generic libs anyway. The difference comes at runtime, when different BLAS / LAPACK implementations are loaded.

[bnavigator]

Shall we drop the Windows Generic builds altogether? conda by default installs MKL, and I guess most people who don't want that use OpenBLAS.

Look at the job results of this PR. The fails are random. This time in the OpenBLAS builds.

[roryyorke]

Thanks @bnavigator . I'm going to open an issue on this, so if anyone has the stamina to setup a Windows development environment they can investigate.