Nightly SNAPSHOT build for stable branches

[megglos]

Description

During a recent 8.6/8.7 patch release we encountered breaking upstream changes (incompatible protobuf dep) that caused the ZPT build to fail. We needed to align the protobuf deps with #2001 to resolve this.

See https://camunda.slack.com/archives/C06UWQNCU7M/p1767684789443219?thread_ts=1767633506.925579&cid=C06UWQNCU7M

In order to detect such incompatibilities that block a release earlier, we should setup nightly stable branch SNAPSHOT builds, with ZPT using the latest upstream SNAPSHOT. Like we have for main with https://github.com/camunda/zeebe-process-test/blob/main/.github/workflows/snapshot-test.yml