ci: also push main tag on release

[nadiamoe]

We don't use mutable tags in production, but it might useful for other repos to pull this in CI/CD tests using a predictable name.

Still unsure whether to go for this approach, or to have renovate update the tag on ci/cd worfklows 🤔

[nadiamoe]

This won't be needed with https://github.com/grafana/sm-k6-runner/pull/232/commits/ff1a5bf59bca7cdb63091bb642f0ea3a226d956d.

Using a immutable tag provides better visibility I think.