Use builder.cargo() for cargo-vendor.

This makes it go through boostrap/bin/rustc.rs, so it will use
-crt-static if needed.

src/bootstrap/dist.rs

@@ -951,13 +951,16 @@ impl Step for PlainSourceTarball {
                 has_cargo_vendor |= line.starts_with("cargo-vendor ");
             }
             if !has_cargo_vendor {
-                let mut cmd = Command::new(&builder.initial_cargo);
-                cmd.arg("install")
-                   .arg("--force")
+                let mut cmd = builder.cargo(
+                    builder.compiler(0, builder.config.build),
+                    Mode::Tool,
+                    builder.config.build,
+                    "install"
+                );
+                cmd.arg("--force")
                    .arg("--debug")
                    .arg("--vers").arg(CARGO_VENDOR_VERSION)
-                   .arg("cargo-vendor")
-                   .env("RUSTC", &builder.initial_rustc);
+                   .arg("cargo-vendor");
                 if let Some(dir) = builder.openssl_install_dir(builder.config.build) {
                     builder.ensure(native::Openssl {
                         target: builder.config.build,