Pull miri from miri branch of dtolnay/rust-toolchain

Author: dtolnay

.github/workflows/ci.yml

@@ -111,9 +111,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: dtolnay/rust-toolchain@nightly
-        with:
-          components: miri
+      - uses: dtolnay/rust-toolchain@miri
       - run: cargo miri test --all-features
 
   clippy: