hash 'CFG_RELEASE_CHANNEL' in metadata for crates

[Keruspe]

This avoids having conflicting hashes between two rustc channels when a
library has not changed

This requires rust-lang/rust#42696 to work properly as part of rustbuild, otherwise the build system can get confused and try to access libraries with a wrong hash.

[rust-highfive]

r? @brson

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[alexcrichton]

Thanks for the PR! I feel though that this is the wrong way to fix the problem. This environment variable is only relevant for rust-lang/rust and not all the other cargo projects? Can a fix be applied to src/bootstrap/bin/rustc.rs instead of Cargo?

[Keruspe]

I'm currently looking into it.
I was blindly focusing on the hash, but maybe modifying src/bootstrap/bin/rustc.rs to insert the channel in the metadata flag would be acceptable?
For example, instead of libbitflags-1199f1aa498b4cd1.rlib we'd have libbitflags-1199f1aa498b4cd1-stable.rlib and libbitflags-1199f1aa498b4cd1-beta.rlib.

If that's an acceptable solution it sure seems to be easily doable