Use system extractor executable

[erri120]

Fixes #1306.

Usage:

NEXUSMODS_APP_USE_SYSTEM_EXTRACTOR=1 dotnet build

[Sewer56]

I'll handle this when I'm back to work tomorrow.
Just gotta remember that there's always the fact we also have to patch the code to invoke system 7zz. i.e. just remove path up to 7zz itself.

[erri120]

Right, then something like USE_SYSTEM_EXTRACTOR might be more fitting.

[Sewer56]

Looks perfectly reasonable to me.
We could also check AppImage just in case, but I doubt that'd be an issue even if used there.
AppImage users probably won't want this flag as by design the AppImage should ship the deps.