Mutable tags like @v6 can be re-pointed by whoever controls the action,
so the next run executes code nobody here reviewed, with the job's
token and secrets. Pinning to the full commit sha freezes what runs.
Version tags are kept as comments so renovate or dependabot can still
track updates.
dtolnay/rust-toolchain is pinned to a master commit with the toolchain
moved to an explicit input, as its readme recommends for sha pinning.
Thibaud-Vdb