Refactor push-to-app-catalog job

[ubergesundheit]

Towards https://github.com/giantswarm/giantswarm/issues/32216#issuecomment-2550805920

This PR cleans up some duplications.

Tested in https://app.circleci.com/pipelines/github/giantswarm/kyverno-policies-connectivity?branch=ubergesundheit-patch-1 for architect based builds and https://app.circleci.com/pipelines/github/giantswarm/external-dns-app?branch=hackathon-test-architect-orb for app-build-suite based builds

Checklist

• ⚠️ Update changelog in CHANGELOG.md.

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[uvegla]

app-operator test: giantswarm/app-operator#1319

[uvegla]

LGTM, thanks a lot! 🙇🏻