Auto merge of #7699 - ehuss:build-std-no-sysroot, r=alexcrichton

Auto merge of #7699 - ehuss:build-std-no-sysroot, r=alexcrichton

Switch build-std to use --extern

Switch `build-std` to use `--extern` flags instead of `--sysroot`.

This is mostly a revert of #7421. It uses the new extern flag options introduced in https://github.com/rust-lang/rust/pull/67074. It avoids modifying the extern prelude which was the source of the problem of https://github.com/rust-lang/wg-cargo-std-aware/issues/40.

Closes https://github.com/rust-lang/wg-cargo-std-aware/issues/49
Reopens https://github.com/rust-lang/wg-cargo-std-aware/issues/31