Cygwin dist succeeds with broken cygffi dependency

[actalley]

Tasks for building cbqn-cygwin-standalone-x86_64 and cbqn-cygwin-mintty-standalone-x86_64 succeed, but BQN.exe and cbqn.dll fail on load with The code execution cannot proceed because cygffi-8.dll was not found. Reinstalling the program may fix this problem.

Tasks PackageCBQNStandalone and PackageCBQNMinttyStandalone in src\cygwin\psakefile.ps1 use file lists in the property section to copy cygffi-6.dll. I'm updating the file list to use cygffi-8.dll, but it should be possible to autodetect dependency name changes and copy the correct files after upstream changes instead of using static lists. Dist tasks should fail if actual dependencies are not met.