Fix jq compile error in cache/resolve keys unknown-key binding

[Copilot]

The entries jq filter in actions/bazel/cache/resolve/action.yml failed to compile (Child process exited with code 5) due to an unparenthesized unique as $unknown binding. This change applies the minimal one-line fix so the filter parses and executes as intended for keys selection and unknown-key validation.

• Root-cause fix (one-line jq syntax correction)

  • Parenthesize the pipeline feeding as $unknown so jq binds the computed array result, not a malformed expression.
  • No changes to lock-hash, resolved, modules, or caches steps; no workflow/test rewrites.

• Behavior preserved

  • Existing keys handling remains intact, including the string-splitting path used for multi-line YAML scalar input.
  • Unknown key detection continues to fail resolution via the existing error path.

- | [$keys[] | . as $key | select($entries | has($key) | not)] | unique as $unknown
+ | ([$keys[] | . as $key | select($entries | has($key) | not)] | unique) as $unknown