Bump nightly version used in CI

Author: rockbmb

.github/workflows/rust-checks.yaml

@@ -13,7 +13,7 @@ concurrency:
 env:
   RUST_BACKTRACE: 1
   # pin nightly to avoid constantly throwing out cache
-  TOOLCHAIN_FMT: nightly-2024-04-09
+  TOOLCHAIN_FMT: nightly-2026-01-01
 
 jobs:
   doc: