Running the release tool with --dry-run should not post PRs

[joestringer]

The flag says:

      --dry-run                   If enabled, it will not perform irreversible operations into GitHub such as: syncing GH projects, pushing tags. All changes that are done locally as well as creating and pushing PRs are also considered reversible and therefore not affected by this flag's value.

Even if posting a PR is reversible, it is surprising for the tool to push a PR when --dry-run is enabled. For instance, I was running this script to locally inspect what the tool would generate, and then it opened a PR on cilium/cilium.

[joestringer]

I would typically expect a dry run to avoid all write operations. I think this is interesting in context of making changes to the filesystem since some of the steps do rely on filesystem changes in order to execute to the next step (for instance, calculating which commits were introduced since the last release).