ci: remove outdated rust-toolchain file

The rust-toolchain file specified Rust 1.79, which is below the minimum
supported version (1.81). When this file exists, `cargo +nightly fmt`
fails on macOS because cargo is invoked directly from the specified
toolchain instead of through rustup, bypassing the +toolchain directive.

Also removes the now-unnecessary git diff exclusion for rust-toolchain.

Author: dannywillems

.github/actions/build-and-test/action.yml

@@ -69,4 +69,4 @@ runs:
     - name: Check that up-to-date bindings are checked in
       shell: bash
       run: |
-        git diff --exit-code ":(exclude)rust-toolchain"
+        git diff --exit-code