scripts: remove +nightly from fmt

The formatting no longer needs nightly and only uses stable features.

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

scripts/ci/fmt

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 set -eoux pipefail
 
-cargo +nightly fmt -- --check
+cargo fmt -- --check