Don't consider bootstrap/defaults to invalidate the rustc cache

Anything that can change there can also be changed in bootstrap.toml.
We have a separate check to make sure that the local config is
compatable with CI's config.

Author: jyn514

src/bootstrap/src/core/config/config.rs

@@ -78,6 +78,7 @@ pub const RUSTC_IF_UNCHANGED_ALLOWED_PATHS: &[&str] = &[
     ":!src/rustdoc-json-types",
     ":!tests",
     ":!triagebot.toml",
+    ":!src/bootstrap/defaults",
 ];
 
 /// Global configuration for the entire build and/or bootstrap.