Use custom sample project to validate toolchain container

[sunjayBhatia]

So we don't run into the issue we had with #117 we could override the default bazel-toolchains sample CC project that is used to build the toolchain configs with one that also emulates a foreign_cc project (with CMake scripts etc.). This would ensure less future errors when modifying the toolchain container.

See: https://github.com/bazelbuild/bazel-toolchains/blob/d1af351b18c800a81677d9b924d4458a742ed8f4/rules/rbe_repo.bzl#L65 (use a sample project we host in this repo)