In order to run package:ffigen on bots, we need libclang.

• ffi plugin template ffigen command doesn't work on Windows #105513
• plugin_ffi template test run ffigen #105688

Without libclang available, package:ffigen cannot find the dynamic library provided by libclang.

Installation instructions: https://github.com/dart-lang/ffigen#installing-llvm

I'm unfamiliar with the setup on our Flutter LUCI bots, so I cannot recommend how to get/package them. Maybe @athomas has suggestions if the bots are sufficiently similar to the Dart bots.

The install location does not matter, default locations are the best. We can add these to package:ffigen and automatically detect it. Alternatively, we can also pass in a path to the test as a commandline argument.