[WIP] Don't require building a stage1 compiler before running cargotest

- Always use a bootstrap compiler for cargo and cargo-credential helpers

  This seems wrong; I think cargo is intentionally built with stage 2
  because it's distributed with other parts of the release. However, I
  don't know how to avoid it - just changing CargoTest to build it with
  stage 0 unconditionally still builds rustc because it uses a `ToolRustc`
  mode.

- Don't call `builder.ensure(Rustc)`. It's unnecessary.
- Don't use the same compiler for building cargo as for testing.

src/bootstrap/test.rs

@@ -167,9 +167,9 @@ impl Step for Cargotest {
     /// This tool in `src/tools` will check out a few Rust projects and run `cargo
     /// test` to ensure that we don't regress the test suites there.
     fn run(self, builder: &Builder<'_>) {
-        let compiler = builder.compiler(self.stage, self.host);
-        builder.ensure(compile::Rustc { compiler, target: compiler.host });
-        let cargo = builder.ensure(tool::Cargo { compiler, target: compiler.host });
+        let bootstrap_compiler = builder.compiler(0, self.host);
+        let cargo = builder
+            .ensure(tool::Cargo { compiler: bootstrap_compiler, target: bootstrap_compiler.host });
 
         // Note that this is a short, cryptic, and not scoped directory name. This
         // is currently to minimize the length of path on Windows where we otherwise
@@ -179,13 +179,14 @@ impl Step for Cargotest {
 
         let _time = util::timeit(&builder);
         let mut cmd = builder.tool_cmd(Tool::CargoTest);
+        let test_compiler = builder.compiler(self.stage, self.host);
         try_run(
             builder,
             cmd.arg(&cargo)
                 .arg(&out_dir)
                 .args(builder.config.cmd.test_args())
-                .env("RUSTC", builder.rustc(compiler))
-                .env("RUSTDOC", builder.rustdoc(compiler)),
+                .env("RUSTC", builder.rustc(test_compiler))
+                .env("RUSTDOC", builder.rustdoc(test_compiler)),
         );
     }
 }

src/bootstrap/tool.rs

@@ -595,7 +595,7 @@ impl Step for Cargo {
 
     fn make_run(run: RunConfig<'_>) {
         run.builder.ensure(Cargo {
-            compiler: run.builder.compiler(run.builder.top_stage, run.builder.config.build),
+            compiler: run.builder.compiler(0, run.builder.config.build),
             target: run.target,
         });
     }
@@ -606,7 +606,7 @@ impl Step for Cargo {
                 compiler: self.compiler,
                 target: self.target,
                 tool: "cargo",
-                mode: Mode::ToolRustc,
+                mode: Mode::ToolBootstrap,
                 path: "src/tools/cargo",
                 is_optional_tool: false,
                 source_type: SourceType::Submodule,
@@ -621,7 +621,7 @@ impl Step for Cargo {
                 compiler: self.compiler,
                 target: self.target,
                 tool: name,
-                mode: Mode::ToolRustc,
+                mode: Mode::ToolBootstrap,
                 path,
                 is_optional_tool: true,
                 source_type: SourceType::Submodule,