Run tests for the cargo submodule in tree

Previously the `cargotest` suite would run some arbitrary revision of Cargo's
test suite, but now that we're bundling it in tree we should be running the
Cargo submodule's test suite instead.

src/bootstrap/lib.rs

@@ -557,6 +557,19 @@ impl Build {
             cargo.env("RUSTC_SAVE_ANALYSIS", "api".to_string());
         }
 
+        // When being built Cargo will at some point call `nmake.exe` on Windows
+        // MSVC. Unfortunately `nmake` will read these two environment variables
+        // below and try to intepret them. We're likely being run, however, from
+        // MSYS `make` which uses the same variables.
+        //
+        // As a result, to prevent confusion and errors, we remove these
+        // variables from our environment to prevent passing MSYS make flags to
+        // nmake, causing it to blow up.
+        if cfg!(target_env = "msvc") {
+            cargo.env_remove("MAKE");
+            cargo.env_remove("MAKEFLAGS");
+        }
+
         // Environment variables *required* needed throughout the build
         //
         // FIXME: should update code to not require this env var