Make JUCE fetching optional. Build macOS universal only in CI.

[sudara]

Supersedes #77

Green build: https://github.com/sudara/pluginval/actions/runs/2690835956

[drowaudio]

Why does a single arch need to be the default? And will this default to arm64 on AS Macs?

[sudara]

Single arch was the default pre-1.0 (Yes, CMake defaults to the build machine's arch). It just means locally people don't have to build (and wait) for two architectures (which is only necessary for distribution)

@eyalamirmusic pointed out it might be better to use an option, but I just left the comment there for clarity (can't think of a great use case for someone wanting to build a universal locally).

[eyalamirmusic]

Why does a single arch need to be the default? And will this default to arm64 on AS Macs?

On IDEs that aren't Xcode, like CLion, VSCode or command line builds, with the universal build flag on, the build will always run for the two architectures which is very slow for development or just local builds.

Also, in my use case of using PluginVal inside another project, that flag will cause all kinds of trouble with my plugin due to it being global, so it's better only set to be universal when needed (like running on a CI, where the build context is fully known), and accept the global flags set by the user, either via command line or via some top-level CMakeLists.txt that might be more informed on the current requirements.