add a Solaris VM test

[Cyan4973]

to Github Actions CI

[Cyan4973]

One issue with the cmake recipe is that it requires the CXX compiler to compile zstd.
This is incorrect. zstd itself doesn't require CXX, only C. This is a problem for systems which can provide a C compiler but not a C++ one, since the cmake recipe will fail them.
It also seems this over-reach has been used and depended upon in some parts of the complex multi-files cmake recipe. Consequently, removing CXX from the zstd project creates errors in some environments,
cmake + visual being one of them.
I guess fixing this is a preliminary step in order to allow Solaris VM as a test.