Include action version in client conformance report

[jku]

It would be nice if client conformance report https://sigstore.github.io/sigstore-conformance/ contained the sigstore-conformance version that was used for each client.

This is a little tricky because

• git describe --tags --always during action run gives us the clients tag, not sigstore-conformance
• it seems variables like GITHUB_ACTION_REF are not available in composite actions

So we may have to include the version in python sources to be able to inject it into the clients conformance-report.json

[jku]

the value of having the action version visible is not that big: humans can always get the action version by looking at run logs or the workflow file at the linked commit...