Use Mode::ToolBootstrap for building book deps

src/bootstrap/src/core/build_steps/test.rs

@@ -2240,7 +2240,7 @@ impl BookTest {
         let libs = if !self.dependencies.is_empty() {
             let mut lib_paths = vec![];
             for dep in self.dependencies {
-                let mode = Mode::ToolRustc;
+                let mode = Mode::ToolBootstrap;
                 let target = builder.config.build;
                 // CHECKME: is this correct, or should it be using `builder::Cargo::new`?
                 let cargo = tool::prepare_tool_cargo(