fix(yarn): no utf for added offline modules

Closes #4126

lib/manager/npm/post-update/index.js

@@ -505,7 +505,7 @@ async function getAdditionalFiles(config, packageFiles) {
                   const localModified = upath.join(config.localDir, f);
                   updatedArtifacts.push({
                     name: f,
-                    contents: await fs.readFile(localModified, 'utf8'),
+                    contents: await fs.readFile(localModified),
                   });
                 }
               }