CI: Exclude import libraries from list of DLLs on Cygwin.

[DWesl]

There are import libraries in numpy.random (with extension .dll.a) that are intended for distribution. The script to list DLLs on Cygwin should not include those, only actual DLLs.

See #24206 for the issue prompting this PR.

[charris]

Looks good, but I am not clear on what the difference does. Could you add a bit of explanation?

[charris]

Thanks @DWesl .

[DWesl]

Import libraries, .dll.a on Cygwin and .lib on WIndows, seem to be a list of functions found in the corresponding shared library (.dll on both), which allows compilers to verify at link time that there is a definition for all symbols, which will be available at run time.