Fix CI: keep stable as default toolchain, use cargo-binstall

Add override: false to nightly toolchain step so stable remains
the default (fixes missing cargo-fmt on nightly). Replace
taiki-e/install-action with cargo-binstall for flexibility.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: akesson

.github/workflows/ci.yml

@@ -16,7 +16,10 @@ jobs:
         with:
           components: clippy, rustfmt
       - uses: dtolnay/rust-toolchain@nightly
-      - uses: taiki-e/install-action@nextest
+        with:
+          override: false
+      - uses: cargo-bins/cargo-binstall@main
+      - run: cargo binstall --no-confirm cargo-nextest
       - run: cargo fmt -- --check
       - run: cargo clippy -- -D warnings
       - run: cargo nextest run