Install scip during GitHub builds

[d-torrance]

macOS only -- available in brew but not apt

[d-torrance]

@mikestillman: It looks like the problem is that we're assuming that scip supports the zimpl reader:

no reader for input file </tmp/M2-10537-0/31/codim.zpl> available

But the brew scip formula turns off zimpl support:

 system "cmake", "-S", ".", "-B", "build", "-DZIMPL=OFF", *std_cmake_args

So I guess we need to use a different format if we want the brew scip to work?

[mikestillman]

I'll take a look and see if changing the format will be easy. It would be good to use built-in binaries...