Fix --dry-run for dist::RustDev

Author: jyn514

src/bootstrap/dist.rs

@@ -1873,7 +1873,7 @@ fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir
         let mut cmd = Command::new(llvm_config);
         cmd.arg("--libfiles");
         builder.verbose(&format!("running {:?}", cmd));
-        let files = output(&mut cmd);
+        let files = if builder.config.dry_run { "".into() } else { output(&mut cmd) };
         let build_llvm_out = &builder.llvm_out(builder.config.build);
         let target_llvm_out = &builder.llvm_out(target);
         for file in files.trim_end().split(' ') {