fix: do not create onboarding branch on dry run (#3333)

This fixes the following scenario:

- on boarding branch is created
- on boarding branch is closed (merged or not) - branch is deleted. So renovate is not configured.
- you relaunch renovate with `--dry-run true`. Then it crashes with:

lib/workers/repository/onboarding/branch/index.js

@@ -27,7 +27,9 @@ async function checkOnboardingBranch(config) {
     logger.info('Need to create onboarding PR');
     await createOnboardingBranch(config);
   }
-  await platform.setBaseBranch(onboardingBranch);
+  if (!config.dryRun) {
+    await platform.setBaseBranch(onboardingBranch);
+  }
   const branchList = [onboardingBranch];
   return { ...config, repoIsOnboarded, branchList };
 }