ci: Remove dead sccache configuration (#13289)
## Why
The AWS-backed sccache setup in CI was disabled a while ago, leaving
commented-out env vars and steps in two workflows. Dead config is noise
that misleads readers into thinking sccache is one uncomment away from
working; the AWS bucket and credentials backing it are no longer in
play.
## What
Removes the commented-out `SCCACHE_*`/`RUSTC_WRAPPER`/AWS env vars and
`mozilla-actions/sccache-action` steps from the `rust_clippy` job
(`lint.yml`) and the `check-lockfiles` job (`turborepo-test.yml`).
`CARGO_INCREMENTAL: 0` stays: disabling incremental compilation remains
the right call in CI even without sccache, since incremental artifacts
add build time with no reuse across jobs.
## How
Comment-only deletions; no behavioral change to CI. Verifiable by
confirming both workflows produce identical runs before and after.
