re-enable new cargo build dir layout testing

Author: RalfJung

src/tools/miri/ci/ci.sh

@@ -28,8 +28,7 @@ begingroup "Building Miri"
 export RUSTFLAGS="-D warnings"
 export CARGO_INCREMENTAL=0
 export CARGO_EXTRA_FLAGS="--locked"
-# FIXME: disabled since it causes libstd build failures
-#export CARGO_UNSTABLE_BUILD_DIR_NEW_LAYOUT=true
+export CARGO_UNSTABLE_BUILD_DIR_NEW_LAYOUT=true
 
 # Determine configuration for installed build (used by test-cargo-miri and `./miri bench`).
 # We use the default set of features for this.