Disable "Bootstrap" test by default (it requires network).

Index: src/bootstrap/test.rs
--- src/bootstrap/test.rs.orig
+++ src/bootstrap/test.rs
@@ -2686,7 +2686,7 @@ pub struct Bootstrap;
 
 impl Step for Bootstrap {
     type Output = ();
-    const DEFAULT: bool = true;
+    const DEFAULT: bool = false;
     const ONLY_HOSTS: bool = true;
 
     /// Tests the build system itself.