bootstrap: Canceling rustc download poisons the cache

I don't know what's different from my other checkouts, but in one of my checkouts, extracting the downloaded rustc is somehow broken:

$ ./x test library/core -- pin
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
extracting /home/r/src/rust/rustc/build/cache/321dde12528a6baf0990ec611d33122b68c33fca-false/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz to /home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/ci-rustc
extracting /home/r/src/rust/rustc/build/cache/321dde12528a6baf0990ec611d33122b68c33fca-false/rustc-nightly-x86_64-unknown-linux-gnu.tar.xz to /home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/ci-rustc

thread 'main' panicked at src/bootstrap/src/core/download.rs:345:17:
member.unpack_in(dst) failed with failed to unpack `/home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/ci-rustc/rustc-nightly-x86_64-unknown-linux-gnu/rustc/lib/librustc_driver-ef85d6b35b33584e.so`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Panic was initiated from src/bootstrap/src/core/download.rs:345:17
Build completed unsuccessfully in 0:00:02

My HEAD is 321dde12528a6baf0990ec611d33122b68c33fca, unchanged. I get the same result with ./x check library --stage 1 so it's not just test; that same command works fine in other checkouts.

The backtrace does not seem like it'd help much:

stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/core/src/panicking.rs:75:14
   2: bootstrap::core::download::<impl bootstrap::core::config::config::Config>::unpack
             at ./src/bootstrap/src/utils/helpers.rs:57:23
   3: bootstrap::core::download::<impl bootstrap::core::config::config::Config>::download_component
             at ./src/bootstrap/src/core/download.rs:694:13
   4: bootstrap::core::download::<impl bootstrap::core::config::config::Config>::download_ci_component
             at ./src/bootstrap/src/core/download.rs:616:9
   5: bootstrap::core::download::<impl bootstrap::core::config::config::Config>::download_toolchain
             at ./src/bootstrap/src/core/download.rs:587:13
   6: bootstrap::core::download::<impl bootstrap::core::config::config::Config>::download_ci_rustc
             at ./src/bootstrap/src/core/download.rs:532:9
   7: bootstrap::core::config::config::Config::download_rustc_commit::{{closure}}
             at ./src/bootstrap/src/core/config/config.rs:1204:21
   8: std::sync::once_lock::OnceLock<T>::get_or_init::{{closure}}
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/once_lock.rs:310:50
   9: std::sync::once_lock::OnceLock<T>::initialize::{{closure}}
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/once_lock.rs:518:19
  10: std::sync::poison::once::Once::call_once_force::{{closure}}
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/poison/once.rs:214:40
  11: std::sys::sync::once::futex::Once::call
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sys/sync/once/futex.rs:176:21
  12: std::sync::poison::once::Once::call_once_force
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/poison/once.rs:214:9
  13: std::sync::once_lock::OnceLock<T>::initialize
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/once_lock.rs:517:9
  14: std::sync::once_lock::OnceLock<T>::get_or_try_init
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/once_lock.rs:396:9
  15: std::sync::once_lock::OnceLock<T>::get_or_init
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/std/src/sync/once_lock.rs:310:15
  16: bootstrap::core::config::config::Config::download_rustc_commit
             at ./src/bootstrap/src/core/config/config.rs:1200:9
  17: bootstrap::core::config::config::Config::download_rustc
             at ./src/bootstrap/src/core/config/config.rs:1190:9
  18: bootstrap::Build::download_rustc
             at ./src/bootstrap/src/lib.rs:314:17
  19: bootstrap::core::builder::StepDescription::run
             at ./src/bootstrap/src/core/builder/mod.rs:468:12
  20: bootstrap::core::builder::Builder::run_step_descriptions
             at ./src/bootstrap/src/core/builder/mod.rs:1232:9
  21: bootstrap::core::builder::Builder::execute_cli
             at ./src/bootstrap/src/core/builder/mod.rs:1212:9
  22: bootstrap::Build::build
             at ./src/bootstrap/src/lib.rs:686:17
  23: bootstrap::main
             at ./src/bootstrap/src/bin/main.rs:99:5
  24: core::ops::function::FnOnce::call_once
             at /rustc/95597e848d27a82b8864c677ab807e9f0be1f68c/library/core/src/ops/function.rs:250:5

Cc @onur-ozkan

Start in src/bootstrap/src/core/download.rs at the unpack failure, then trace the download flow through src/bootstrap/src/core/config/config.rs and src/bootstrap/src/utils/helpers.rs. Reproduce with ./x test library/core -- pin or ./x check library --stage 1 and inspect behavior after canceling the rustc download. Done means an interrupted download no longer leaves the cache unusable for later runs.