ci: remove -Dwarnings from coverage nightly build

Only enable -Dwarnings in stable toolchain as in build and test.

Author: kezhuw

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
     needs: [test]
     runs-on: ubuntu-latest
     env:
-      RUSTFLAGS: -Cinstrument-coverage -Dwarnings
+      RUSTFLAGS: -Cinstrument-coverage
     steps:
     - uses: actions/checkout@v2
     - uses: Swatinem/rust-cache@v1