Use CPack with `$<TARGET_RUNTIME_DLLS>` to avoid hard-coding .dll filenames

[Rangi42]

As per madler/zlib#1181 (comment), the name of zlib1.dll has changed to z.dll, and in future will change to libz.dll. (And separate packages like libz-mingw-w64-dev may continue to use older names.)

We currently hard-code the .dll names in our testing and create-release-artifacts workflows, in order to package and upload them to GitHub. So keeping the .dll names up-to-date can be inconvenient, as in 65b0b49.

It might be better to use CPack with $<TARGET_RUNTIME_DLLS> to automatically pack the appropriate files via a CMake command, so zlib's own build system would automatically handle the correct name.

This would replace the Compress-Archive run command in the create-release-artifacts workflow. It would also change the actions/upload-artifact action in the testing workflow to upload a single already-zipped file.