Install bindgen in the build environment

[jyn514]

I tested locally that this only installs bindgen when new versions are
published, not on every build.

Fixes #444.

r? @Nemo157

[Nemo157]

rust-lang/cargo#5730 is fixed with the new resolver, and nrfxlib-sys has updated to rely on that, are there any other crates still relying on this? (Eventually rust-lang/rfcs#3028 would allow having a build-dependency on the bindgen binary if for some reason crates want to call it as a CLI instead of a library).

[jyn514]

Hmm, that's a good point - I'd forgotten the original reason they used the binary.

I don't know any way we could check this. I guess we can close the issue and see if anyone complains?

[Nemo157]

Yeah, the lack of "me too" comments on the issue makes me think it's a pretty rare problem, and now that it's fixed upstream everyone should hopefully migrate to resolver = "2".