Harden the composite action and testing workflow against
High-severity and Medium-severity zizmor findings:

- template-injection: route ${{ inputs.* }} and other expansions
  through step-level env: blocks and reference them as quoted
  shell variables, preventing expression injection into run:
  scripts.
- ref-version-mismatch: correct stale version comments on pinned
  actions (setup-uv -> v8.2.0, actions/cache -> v5.0.5) so they
  match the commit the SHA actually points to.
- github-env: the values persisted to GITHUB_ENV are action-
  internal and validated before being written (the action already
  guards against env-file injection); they are env-by-nature and
  carry scoped, justified ignores.

Also address review feedback:

- drop the redundant INPUTS_* env entries an autofix introduced
  (including INPUTS_X, an undeclared input). The steps consume the
  INPUT_* variables; the duplicate INPUTS_* names were unused and
  confusing.
- reword the in-script justification comments to name the GitHub
  Actions expression form (e.g. inputs.path_prefix) instead of
  bash-style ${INPUTS_X} placeholders. The literal ${{ ... }}
  token is deliberately avoided inside run: comments because the
  runner would interpolate it -- the very vector being mitigated.

Confirmed clean with zizmor (0 High/Medium findings remaining).

Co-authored-by: Claude <claude@anthropic.com>
Signed-off-by: Matthew Watkins <mwatkins@linuxfoundation.org>