[7.17](backport #42326) Don't run packaging snapshot with prereleases

[mergify]

Proposed commit message

This fixes a small bug related to #42304 (example failure link).
When we (manually) trigger prerelease builds (by setting VERSION_QUALIFIER="...") we only want them to trigger staging builds and skip snapshot builds.

Related issues

https://github.com/elastic/ingest-dev/issues/4857

---

This is an automatic backport of pull request #42326 done by [Mergify](https://mergify.com).

[mergify]

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