fix: declare rustfmt aspect action toolchain (#4128)

The --incompatible_auto_exec_groups flag requires Starlark actions to
declare whether their executable/tools come from an action toolchain.
This is not a Bazel default today, but it is exposed through strict
preset configurations and is intended to catch rules that rely on
implicit action toolchain inference.

rustfmt_aspect runs rustfmt from rules_rust's rustfmt toolchain, but the
formatting action does not declare that action toolchain. Declare the
matching rustfmt toolchain so the action's toolchain behavior is
explicit without changing its inputs or execution model.