DownloadManager builds by newer GHA Windows runners fail to load

[arch1t3cht]

On recent GHA Windows runners, the libcurl wrap picks up zlib and libSSH2, and dynamically links against them. The resulting DLL will then fail to be loaded by LuaJIT on any machine that doesn't have these libraries installed.

Unfortunately, there's no working CI build of ffi-experiments that's still preserved, but the "Generate Windows Installer" step in the following two Aegisub builds also show this:

• Good: https://github.com/arch1t3cht/Aegisub/actions/runs/3510878855
  Logs also archived here: logs_53.zip
• Bad: https://github.com/arch1t3cht/Aegisub/actions/runs/3543940997
  Logs also archived here: logs_54.zip

Disabling zlib and SSH2 support in curl via CMake options (see arch1t3cht@2618e45) solves this, though there might be some better and more general solution that just disables or hides all these optional dependencies.

[arch1t3cht]

I can no longer reproduce this on the CI, so I'm guessing this was fixed in the runner image now. The paths searched by CMake also resemble those in the "good" run again.