build: bootstrap panics when using the --llvm-root option

[filnet]

Issue

Building rust with the --llvm-root option fails with a "missing llvm-tools" error.

Reverting 23a34c1 fixes the issue.

The install llvm-tools step should be skipped if no tarball is provided.
This happens when --llvm-root is used (i.e. externally provided llvm).

Version it worked on

It most recently worked on: Rust 1.67.1

Version with regression

Rust 1.68.0

Backtrace

Backtrace

thread 'main' panicked at 'missing llvm-tools', install.rs:215:14
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_display
   3: core::panicking::panic_str
   4: core::option::expect_failed
   5: core::option::Option<T>::expect
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\library\core\src\option.rs:786:21
   6: <bootstrap::install::LlvmTools as bootstrap::builder::Step>::run
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\install.rs:213:23
   7: bootstrap::builder::Builder::ensure
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\builder.rs:1966:23
   8: <bootstrap::install::LlvmTools as bootstrap::builder::Step>::make_run
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\install.rs:150:17
   9: bootstrap::builder::StepDescription::maybe_run
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\builder.rs:297:13
  10: bootstrap::builder::StepDescription::run
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\builder.rs:334:21
  11: bootstrap::builder::Builder::run_step_descriptions
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\builder.rs:893:9
  12: bootstrap::builder::Builder::execute_cli
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\builder.rs:873:9
  13: bootstrap::Build::build
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\lib.rs:748:17
  14: bootstrap::main
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\src\bootstrap\bin\main.rs:50:5
  15: core::ops::function::FnOnce::call_once
             at C:\M\mingw-w64-rust\src\rustc-1.68.0-src\library\core\src\ops\function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
Build completed unsuccessfully in 0:00:32

[filnet]

I filed a somewhat similar regression yesterday : #109103

Related: e5c92bc

[filnet]

fixed by #109256