fix: don’t apply custom branchPrefix when onboarding

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

@@ -22,7 +22,7 @@ If you have any questions, try reading our [Docs](https://renovateapp.com/docs),
 async function ensureOnboardingPr(config) {
   logger.debug('ensureOnboardingPr()');
   logger.trace({ config });
-  const onboardingBranch = `${config.branchPrefix}configure`;
+  const onboardingBranch = `renovate/configure`;
   const onboardingPrTitle = 'Configure Renovate';
   logger.debug('Filling in onboarding PR template');
   let prBody = prTemplate;
@@ -34,9 +34,7 @@ async function ensureOnboardingPr(config) {
   logger.trace('prBody:\n' + prBody);
 
   // Check if existing PR exists
-  const existingPr = await platform.getBranchPr(
-    `${config.branchPrefix}configure`
-  );
+  const existingPr = await platform.getBranchPr(`renovate/configure`);
   if (existingPr) {
     // Check if existing PR needs updating
     if (existingPr.title === onboardingPrTitle && existingPr.body === prBody) {