rustbuild: Fix test "test rustdoc" invocation

Previously it would use the librustc output directory which would cause rustdoc
to get entirely recompiled, whereas the intention is that it uses the
already-compiled artifacts from building rustdoc itself, using the tool output
directory

src/bootstrap/check.rs

@@ -1118,10 +1118,11 @@ impl Step for Rustdoc {
  let compiler = builder.compiler(builder.top_stage, self.host);
  let target = compiler.host;
 
- builder.ensure(RemoteCopyLibs { compiler, target });
-
- let mut cargo = builder.cargo(compiler, Mode::Librustc, target, test_kind.subcommand());
- compile::rustc_cargo(build, &compiler, target, &mut cargo);
+ let mut cargo = tool::prepare_tool_cargo(builder,
+ compiler,
+ target,
+ test_kind.subcommand(),
+ "src/tools/rustdoc");
  let _folder = build.fold_output(|| {
  format!("{}_stage{}-rustdoc", test_kind.subcommand(), compiler.stage)
  });

src/bootstrap/tool.rs

@@ -94,20 +94,21 @@ impl Step for ToolBuild {
  let _folder = build.fold_output(|| format!("stage{}-{}", compiler.stage, tool));
  println!("Building stage{} tool {} ({})", compiler.stage, tool, target);
 
- let mut cargo = prepare_tool_cargo(builder, compiler, target, path);
+ let mut cargo = prepare_tool_cargo(builder, compiler, target, "build", path);
  build.run(&mut cargo);
  build.cargo_out(compiler, Mode::Tool, target).join(exe(tool, &compiler.host))
  }
 }
 
-fn prepare_tool_cargo(
+pub fn prepare_tool_cargo(
  builder: &Builder,
  compiler: Compiler,
  target: Interned<String>,
+ command: &'static str,
  path: &'static str,
 ) -> Command {
  let build = builder.build;
- let mut cargo = builder.cargo(compiler, Mode::Tool, target, "build");
+ let mut cargo = builder.cargo(compiler, Mode::Tool, target, command);
  let dir = build.src.join(path);
  cargo.arg("--manifest-path").arg(dir.join("Cargo.toml"));
 
@@ -283,6 +284,7 @@ impl Step for Rustdoc {
  let mut cargo = prepare_tool_cargo(builder,
  build_compiler,
  target,
+ "build",
  "src/tools/rustdoc");
  build.run(&mut cargo);
  // Cargo adds a number of paths to the dylib search path on windows, which results in