Show enabling of nightly tests as a CI step

Author: dtolnay

.github/workflows/ci.yml

@@ -24,10 +24,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.56.0]
-        include:
-          - rust: nightly
-            rustflags: --cfg async_trait_nightly_testing
+        rust: [nightly, beta, stable, 1.56.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v3
@@ -37,9 +34,10 @@ jobs:
       - name: Enable type layout randomization
         run: echo RUSTFLAGS=${RUSTFLAGS}\ -Zrandomize-layout >> $GITHUB_ENV
         if: matrix.rust == 'nightly'
+      - name: Enable nightly-only tests
+        run: echo RUSTFLAGS=${RUSTFLAGS}\ --cfg=async_trait_nightly_testing >> $GITHUB_ENV
+        if: matrix.rust == 'nightly'
       - run: cargo test
-        env:
-          RUSTFLAGS: ${{matrix.rustflags}} ${{env.RUSTFLAGS}}
 
   msrv:
     name: Rust 1.39.0