Delete the `stage1` and `stage0-sysroot` directories when using download-rustc

[jyn514]

@camelid: I just ran into something confusing with download-rustc: normal x.py build will build rustdoc and put it in stage1. However, x.py build with download-rustc enabled will put it in stage2. This means that rustdoc +stage1 ... won't run my local, modified version of rustdoc if download-rustc is set. Specifically, rustdoc +stage1 ... seems to run nightly rustdoc (or something static).

@bjorn3 suggests deleting the stage1 and stage0-sysroot directories when using download-rustc, to avoid caching issues like this. rustdoc +stage1 still won't do the right thing, but it will at least give a hard error instead of silently using the wrong version.

Originally posted by @jyn514 in #81930 (comment)

[jyn514]

Mentoring instructions: in https://github.com/rust-lang/rust/blob/405f0c21608cb41aabc512eed73b2ff707391cab/src/bootstrap/compile.rs#L1115-L1126 delete the sysroots stage1 and stage0-sysroot folders, not just the current sysroot.

Some background info: https://rustc-dev-guide.rust-lang.org/building/bootstrapping.html#what-is-a-sysroot

[chenyukang]

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