Properly inform cargo that build scripts look at some env variables

[nox]

https://github.com/servo/webxr/blob/ee9caca02a68195ab7fe54b263619f5af1d0e853/webxr/build.rs#L56

Those things need to be told to cargo, with some cargo:rerun-if-env-changed=… output lines.

[msub2]

@jdm Can also close this one now that ML backend is removed.