schema check baseRef should not be hard-coded

- https://github.com/GoogleContainerTools/skaffold/blob/88e1c1734ea0f1bd79a0f24783f9565057a0e46a/hack/versions/pkg/schema/check.go#L31 is hard-coded to `origin/main`, this causes schema check fail due to the pull request ref doesn't have schema exist on `origin/main`, the check should be against a target ref that the source ref want to merge in not hard-coded to origin/main
- We may use some github action environment variable to get the base ref and then do the comparison.
- some context https://docs.github.com/en/actions/learn-github-actions/contexts#env-context
- looks like we've already had base_sha , pr_sha injected into env https://github.com/GoogleContainerTools/skaffold/blob/2ea2f08b664b9e45d98df1e6e56d702719e15a2f/.github/workflows/linters-checks.yml#L28-L31
- we may just need to update the base ref to use the env vars