--cap-lints should be set for sysroot builds

[RalfJung]

Cargo automatically sets --cap-lints=allow when building dependencies to make sure that e.g. deny(warnings) in a dependency does not break the build. The same should happen when xargo builds the sysroot. xargo has to take care of this itself as cargo cannot know that e.g. the build of std actually is just building a dependency.

[RalfJung]

Once rust-lang/cargo#4423 is fixed, xargo could use RUSTFLAGS (or whatever new environment variable will control build scripts) to set --cap-lints=allow -- a rather straight-forward patch. Until then though it seems we could only set --cap-lints for the actual libraries that cargo compiles for xargo, but not for the build scripts. (And that doesn't help with my current problem, which involves build scripts failing to build.)

[japaric]

--cap-lints should be set for sysroot builds

Sounds good to me.