Cargo on beta needs updating to build Rust on master

[alexcrichton]

In #39463 I've opted due to not upgrade Cargo yet due to the errors preventing landing.

We need to track down what's causing this problem, land that change on Cargo master, then backport that change to rust-1.16.0 Cargo, and then update the Cargo used to bootstrap Rust on the master branch.

I'm tagging this as a regression as it's something we'll need to fix before the next release, and we shouldn't lose track of it!

[alexcrichton]

So here's what I'm thinking is happening

• Cargo emits $folder\deps\libcompiler_builtins-$hash.rlib
• Cargo hard-links that to $folder\libcompiler_builtins.rlib
• Rustbuild hard-links the former to $sysroot/lib/.../libcompiler_builtins-$hash.rlib
• Later on we run cargo test
• As part of cargo test's normal execution, it attempts to re-hard-link the $hash file to the non-deps file (the first two steps above)
• Presumably some concurrent invocation of rustc has the library open (to read it)

This means that we're deleting a hard link where the actual file has a handle open to it elsewhere (opened through another link). Windows doesn't like this!