ci: install nightly before running miri

Author: wyfo

.github/workflows/ci.yml

@@ -19,6 +19,7 @@ jobs:
         run: cargo clippy --all-features --tests -- -D warnings
       - name: test
         run: cargo test --all-features
+      - run: rustup install nightly
       - name: miri
         run: cargo +nightly miri test --all-features --all-seeds
       - name: loom