The cpp default toolchain on windows was hardset to cc-compiler-k8, which
is linux exclusive. Additionally it's quite common to need to change the
compiler being used.

The drawback of this approach is that it potentially creates many repetitive
equal cc_toolchains if you use the same container with multiple toolings installed.

Long term we could add multiple cpp toolchains to configs/, but so far the limited
amount of repetition isn't harmful.