Convert cxxbridge-cmd dependency to bindep

[dtolnay]

This is how I tested #2229.

I am not sure whether we want this in rules_rust yet because the existing @cxxbridge-cmd serves as a useful example for users to copy into their own workspaces, and not all of those users use nightly. If we are open to using -Zbindeps unconditionally on all toolchains (via RUSTC_BOOTSTRAP=1) then I'll finish this up, otherwise defer to after artifact dependencies is stable.

[illicitonion]

Thanks for putting this together!

I think in the short term we should probably block on stabilisation of bindeps, but if that looks like it's going to take forever we can reconsider an approach.

Ideally we would not add RUSTC_BOOTSTRAP=1 in rules_rust :)

[dtolnay]

Okay! That sounds good to me.