Make nightly toolchain used for CI the default

.github/workflows/ci.yml

@@ -3,6 +3,8 @@ name: CI
 env:
   CARGO_TERM_COLOR: always
   CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
+  # Keep in sync with version in `rust-toolchain.toml` and `xtask/src/ci.rs`
+  NIGHTLY: nightly-2023-07-03
 
 on:
   push:
@@ -43,7 +45,7 @@ jobs:
       - name: Install rust nightly toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly-2023-07-03
+          toolchain: ${{ env.NIGHTLY }}
 
       - uses: Swatinem/rust-cache@v2
 
@@ -186,7 +188,7 @@ jobs:
       - name: Install rust nightly toolchain
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly-2023-07-03
+          toolchain: ${{ env.NIGHTLY }}
           components: ${{ matrix.components }}
 
       - uses: Swatinem/rust-cache@v2
@@ -218,7 +220,7 @@ jobs:
         # Nightly needed for use of unstable options
         uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: nightly-2023-07-03
+          toolchain: ${{ env.NIGHTLY }}
 
       - uses: Swatinem/rust-cache@v2