make CC env variable configurable

[andrewwillette]

I hope all is well!

This is to address issues I ran into running tests on macOS with runtime/cgo dependencies. The default C compiler appears to be gcc-11, I required clang for my arm64 architecture.

The related build failures in the neotest-go workflow are not simple to troubleshoot.

I made an effort to utilize the RunSpec env configuration with it's subsequent env configuration in the runners pcall, but passing the configured CC there does not correctly configure the execution in the go test process.