* fix: add 'version' input to GitHub action

This allows specifying a GitHub action version independent from the
conformance test (client) version and fixes the issue where conformance tests are being
updated even when specifying an older Action release.

Note, that GitHub Actions recommends placing the Action in a separate
repo, so it may be worth moving `/actions` into its own repo at some
point: https://docs.github.com/en/actions/creating-actions/about-custom-actions#choosing-a-location-for-your-action