use HEAD ref in [github] fetchJsonFromRepo (#5589)

Co-authored-by: repo-ranger[bot] <39074581+repo-ranger[bot]@users.noreply.github.com>

services/github/github-common-fetch.js

@@ -55,7 +55,7 @@ async function fetchRepoContent(
 
 async function fetchJsonFromRepo(
   serviceInstance,
-  { schema, user, repo, branch = 'master', filename }
+  { schema, user, repo, branch = 'HEAD', filename }
 ) {
   if (serviceInstance.staticAuthConfigured) {
     const buffer = await fetchRepoContent(serviceInstance, {