fix: do not copy cypress binary on macos

[jbedard]

Fixes #43

[alexeagle]

I guess there's some reason we must continue to copy it on non-Macos platforms??

[mrmeku]

I guess there's some reason we must continue to copy it on non-Macos platforms??

Yeah, directories in a workspace's source tree aren't compatible with RBE. Thus we copy the directory to bazel-out for RBE support.

This change essentially means that RBE will be supported only on linux platforms. I don't love that that's necessary, but I don't see any other way around the mac security prompt

[gregmagolan]

@mattem recently got RBE working with directories source trees with RBE with our diff_test rule tho I don't remember how he did it