change CI rustfmt to MSRV toolchain instead of nightly

[alejandro-vaz]

in .github/workflows/rust.yml change the style check to use MSRV instead of nightly

current workflow:

      - name: Style check
        if: matrix.nightly
        run: rustup component add rustfmt && cargo fmt --all --check

this removes formatting issues related to rustfmt updates such as in #539