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

Index: src/bootstrap/src/core/build_steps/test.rs
--- src/bootstrap/src/core/build_steps/test.rs.orig
+++ src/bootstrap/src/core/build_steps/test.rs
@@ -48,7 +48,7 @@ pub struct CrateBootstrap {
 
 impl Step for CrateBootstrap {
     type Output = ();
-    const IS_HOST: bool = true;
+    const IS_HOST: bool = false;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         // This step is responsible for several different tool paths.
@@ -64,7 +64,7 @@ impl Step for CrateBootstrap {
     }
 
     fn is_default_step(_builder: &Builder<'_>) -> bool {
-        true
+        false
     }
 
     fn make_run(run: RunConfig<'_>) {
