ci: try to fix "Ensure compatibility with rust-gpu nightly rust version" check.

Author: eddyb

.github/workflows/ci.yml

@@ -21,7 +21,9 @@ jobs:
           # FIXME(eddyb) `style_edition = "2024"` in `rustfmt.toml` is what keeps
           # this on `nightly`, should switch to `stable` when that stabilizes.
           toolchain: nightly
-          override: true
+          # HACK(eddyb) allow `+nightly` to work without breaking the ability
+          # to use `rust-toolchain.toml` later.
+          override: false
 
       # run cargo fetch w/ --locked to verify Cargo.lock is up-to-date
       - run: cargo fetch --locked
@@ -30,7 +32,7 @@ jobs:
       - name: check rustfmt
         run: |
           rustup component add rustfmt
-          cargo fmt -- --check --color always
+          cargo +nightly fmt -- --check --color always
 
       # run clippy to verify we have no warnings
       - name: cargo clippy