CI: Keep OpenBLAS on Cygwin
CI: Keep OpenBLAS on Cygwin

This was removed a few months ago to avoid failures with a previous version (#20654, #20660), necessitating the use of the Netlib reference BLAS instead (#20669).  The Cygwin OpenBLAS implementation has updated to 0.3.20 since then (https://cygwin.com/cgi-bin2/package-grep.cgi?grep=openblas&arch=x86_64), so using OpenBLAS again should be fine.
anntzer/numpy,charris/numpy,mattip/numpy,endolith/numpy,charris/numpy,rgommers/numpy,anntzer/numpy,rgommers/numpy,anntzer/numpy,mhvk/numpy,mhvk/numpy,mhvk/numpy,mhvk/numpy,numpy/numpy,charris/numpy,mattip/numpy,mattip/numpy,endolith/numpy,anntzer/numpy,endolith/numpy,rgommers/numpy,mhvk/numpy,mattip/numpy,numpy/numpy,endolith/numpy,numpy/numpy,charris/numpy,rgommers/numpy,numpy/numpy