Pin CI to nightly-2020-07-21 to avoid rust-lang/rust#74614

Author: dtolnay

.github/workflows/ci.yml

@@ -13,6 +13,8 @@ jobs:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@nightly
         with:
+          # Pinned because of https://github.com/rust-lang/rust/issues/74614
+          toolchain: nightly-2020-07-21
           components: rustc-dev
       - run: cargo test --all-features --release