Make nightly 'rustc' feature test work

Author: phansch

.github/workflows/rust.yml

@@ -51,7 +51,9 @@ jobs:
         profile: minimal
         toolchain: nightly
         override: true
+    - name: Add rustc-dev component
+      run: rustup component add rustc-dev
 
     - name: Run test-project tests with `rustc` feature
-      run: cd test-project && cargo test --features rustc
+      run: cd test-project && cargo +nightly test --features rustc