Don't copy tool dependencies to the sysroot

This fixes the following error:

```
error: found crates (`serde_derive` and `serde_derive`) with colliding StableCrateId values.
   --> /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/cargo_metadata-0.8.2/src/lib.rs:162:1
    |
162 | extern crate serde_derive;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
```

This is a bug in resolve (rust-lang#56935)
but it will be difficult to fix in the near future. This works around it
in the meantime by not copying serde_derive and other dependencies to
the sysroot when they're built for other tools. This rebuilds the
dependencies slightly more often than necessary, but avoids the crate
conflicts.

This can be reverted once rust-lang#56935 is fixed.

Author: jyn514

src/bootstrap/check.rs

@@ -342,10 +342,6 @@ macro_rules! tool_check_step {
                     true,
                 );
 
-                let libdir = builder.sysroot_libdir(compiler, target);
-                let hostdir = builder.sysroot_libdir(compiler, compiler.host);
-                add_to_sysroot(&builder, &libdir, &hostdir, &stamp(builder, compiler, target));
-
                 /// Cargo's output path in a given stage, compiled by a particular
                 /// compiler for the specified target.
                 fn stamp(