Fix for issue with private key no longer working as a buffer

So with the bump of @octokit/auth-app from v6.* -> v7.* it bumped its
own dependency of universal-github-app-jwt from v1.* -> v2.*

The universal-github-app-jwt v2 has introduced some work that replaces
newlines within private keys - https://github.com/gr2m/universal-github-app-jwt/blob/d52064d3395a02363b6f80afeeb5d91e0a5f2040/index.js#L17

This work looks like it came in in v2.1.1 - gr2m/universal-github-app-jwt#96

Essentially what this means is with the latest version the privateKey
must be a string rather than a buffer.

This code fixes this issue

src/helpers/oktokit.js

@@ -9,7 +9,10 @@ import { graphql as octokitGraphql } from '@octokit/graphql'
 
 const auth = {
   appId: config.get('github.app.id'),
-  privateKey: Buffer.from(config.get('github.app.privateKey'), 'base64'),
+  privateKey: Buffer.from(
+    config.get('github.app.privateKey'),
+    'base64'
+  ).toString('utf8'),
   installationId: config.get('github.app.installationId')
 }