ci: use rustup toolchain install with --allow-downgrade for nightly rustfmt

rustup component add does not support --allow-downgrade; the flag is only
available on rustup toolchain install. This picks the most recent nightly
that ships rustfmt when the latest nightly lacks it.

Author: bug-ops

.github/workflows/ci.yml

@@ -31,8 +31,7 @@ jobs:
       contents: read
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # actions/checkout v6
-      - uses: dtolnay/rust-toolchain@5b842231ba77f5c045dba54ac5560fed2db780e2 # dtolnay/rust-toolchain nightly
-      - run: rustup component add rustfmt --toolchain nightly --allow-downgrade
+      - run: rustup toolchain install nightly --component rustfmt --allow-downgrade
       - run: cargo +nightly fmt --all -- --check
 
   clippy: