Don't run dryrun when building pr images (#131)

### 📖 Summary

Both essentially do the same until the very end - only one of each is needed respectively.

### 📑 Build PR Images?

PR images are not needed

### 💬 Details

_No response_

Reviewed-on: https://codeberg.org/woodpecker-plugins/docker-buildx/pulls/131
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: pat-s <patrick.schratz@gmail.com>
Co-committed-by: pat-s <patrick.schratz@gmail.com>

.woodpecker.yml

@@ -58,6 +58,7 @@ steps:
       repo: test
       dry-run: true
     when:
+      evaluate: 'not (CI_COMMIT_PULL_REQUEST_LABELS contains "build_pr_images")'
       event: pull_request
       branch:
         - ${CI_REPO_DEFAULT_BRANCH}