GitHub actions: CMake is preinstalled. CPU count is fixed. (#495)
GitHub actions: CMake is preinstalled. CPU count is fixed. (#495)

* CMake is already installed on GitHub Action runners.

* GitHub Actions runners has 2 CPU.
CoolmanCZ/usrsctp,sctplab/usrsctp,sctplab/usrsctp,CoolmanCZ/usrsctp,sctplab/usrsctp,weinrank/usrsctp,weinrank/usrsctp,weinrank/usrsctp,CoolmanCZ/usrsctp,CoolmanCZ/usrsctp