Add PR write permissions for sync-with-master

[prakashsurya]

No description provided.

[prakashsurya]

@sdimitro I don't know how to test this.... do you?

[prakashsurya]

I'm thinking this might be needed based on this document, the fact that we're getting a 403 error currently:

 + hub pull-request -b 6.0/stage -h master -m 'Merge branch '\''master'\'' into '\''6.0/stage'\'''
Error creating pull request: Forbidden (HTTP 403)

and the fact that we appear to be using read permissions:

GITHUB_TOKEN Permissions
  Contents: read
  Metadata: read

Beyond that, though, I'm sort of guessing that this might help fix the issue.