Rollup merge of #98798 - jyn514:download-rustc-cached, r=Mark-Simulacrum

Rollup merge of #98798 - jyn514:download-rustc-cached, r=Mark-Simulacrum

Fix caching bug in `download-rustc = true`

When moving this to rustbuild, I introduced a bug: if you had the file already downloaded, but
deleted the sysroot for whatever reason, rustbuil would fail to unpack the cached tarball.

This only affects people if they have a cached tarball, which is probably why we haven't seen an issue yet -
wiping `build/cache` would work around the issue, or just not deleting `build/$TARGET/stage2`.

Fixes the following error:
```
thread 'main' panicked at 'fs::read_dir(&lib_dir) failed with No such file or directory (os error 2) ("/home/jnelson/rust-lang/rust2/build/x86_64-unknown-linux-gnu/ci-rustc/lib")', config.rs:1563:20
```

r? ``@Mark-Simulacrum``