fix(cache): lazy fetch tags for changelog

lib/workers/pr/changelog/source-github.js

@@ -95,9 +95,12 @@ async function getChangeLogJSON({
     return null;
   }
 
-  const tags = await getTags(config.endpoint, versionScheme, repository);
+  let tags;
 
-  function getRef(release) {
+  async function getRef(release) {
+    if (!tags) {
+      tags = await getTags(config.endpoint, versionScheme, repository);
+    }
     const tagName = tags.find(tag => equals(tag, release.version));
     if (tagName) {
       return tagName;