This prevents the included test case from getting at nightly-only
features when run on stable. The harness builds with
RUSTC_BOOTSTRAP, but the bundle doesn't.
Fixes#132203
This is a compatibility hack, because I think the new behavior is better.
When an A `include_str!` B, and B `include_str!` C, the path to C should
be resolved relative to B, not A. That's how `include!` itself works,
so that's how `include_str!` with should work.
