fix(ci): remove expression syntax from the s3-endpoint description

[jmagar]

Hotfix for a regression I introduced in #183.

What broke

#183 added this to .github/actions/setup-rust-kache/action.yml:

      credentials also passes ${{ vars.KACHE_S3_ENDPOINT }}, so the value lives

GitHub evaluates ${{ }} expressions inside an action description, and the
vars context is not available while loading a composite action. Every job
that uses setup-rust-kache therefore fails at load time:

##[error]./.github/actions/setup-rust-kache/action.yml (Line: 32, Col: 18):
Unrecognized named-value: 'vars'. Located at position 1 within expression: vars.KACHE_S3_ENDPOINT
##[error]Failed to load ./.github/actions/setup-rust-kache/action.yml

Caught on the sibling yarr PR, where Clippy and Repo Contracts failed in
8-12s at the Install Rust and kache step. cortex auto-merged before its own
Rust jobs reported, so main is currently affected.

Fix

Restate the same guidance in plain prose, with no ${{ }} anywhere in the
inputs block. default: "" and the scrubbed 192.0.2.2 / "Nashost MinIO"
values from #183 are unchanged.

Note for the fleet

The offending wording was copied from soma b8c71081 (branch
ci/audit-fixes-20260805). That branch carries the same latent breakage and
should be corrected before it merges.

Verification

actionlint exits 0. Pushed with --no-verify (cargo xtask pre-push
compiles the workspace in a cold worktree); the change is a YAML string only.