rust 1.68.0 install fails with `missing llvm-tools` if using external llvm

[jpalus]

Close to the end of dist target there's:

Skipping LlvmTools (aarch64-unknown-linux-gnu): external LLVM                

but shortly after install fails with:

thread 'main' panicked at 'missing llvm-tools', install.rs:215:14                                                                                                                              

[jpalus]

Looks like 23a34c1 lacks checks for external LLVM:

rust/src/bootstrap/install.rs

Lines 212 to 217 in 2c8cc34

	LlvmTools, alias = "llvm-tools", Self::should_build(_config), only_hosts: true, {
	let tarball = builder
	.ensure(dist::LlvmTools { target: self.target })
	.expect("missing llvm-tools");
	install_sh(builder, "llvm-tools", self.compiler.stage, Some(self.target), &tarball);
	};

like dist does:

rust/src/bootstrap/dist.rs

Lines 1992 to 2001 in 2c8cc34

	fn run(self, builder: &Builder<'_>) -> Option<GeneratedTarball> {
	let target = self.target;
	/* run only if llvm-config isn't used */
	if let Some(config) = builder.config.target_config.get(&target) {
	if let Some(ref _s) = config.llvm_config {
	builder.info(&format!("Skipping LlvmTools ({}): external LLVM", target));
	return None;
	}
	}