Starting from rust-lang/rust#128534 (nightly-2024-08-05), stnadard library has its own Cargo workspace. Hence `-Zbuild-std` no longer need to fake a virtual workspace. This also adjusts Cargo.toml in `mock-std` to align with std's Cargo.toml
libsysroot
libtest
This reverts commit eadb4fc86c6660760ff857df41b7e88f6da0f714.
This is the cargo side of https://github.com/rust-lang/wg-cargo-std-aware/issues/23