Drop the rustup-specific calling in `ci/rustfmt.sh`

Drop the rustup-specific calling in `ci/rustfmt.sh`

The +rustversion call semantics are specific to rustup and do not
work with standard rust toolchains. However, because rustfmt
formatting differs slightly between stable and our 1.63 target, we
need to keep the +1.63.0 for rustup users.

Thus, here, we check for the presence of a `rustup` command and
pass the "+1.63.0" argument if we find one.