dtolnay/rust-toolchain moved from rolling stable/nightly branches to version
tags, so the pinned (still valid, still immutable) SHA is no longer reachable
from a branch and zizmor flags it as an impostor-commit. Add the same
`# zizmor: ignore[impostor-commit]` marker the shazam/copilot repos already
use. Comment-only; the action SHA is unchanged.
kingchenc