Use authenticated GitHub API calls

The staging job is often failing with this error:
```
Pushing kernel test branch for mainline
path: checkout/linux
repo: kernelci/linux
Traceback (most recent call last):
File "/home/kernelci/kernelci-deploy/./kernel.py", line 87, in
ret = main(args)
File "/home/kernelci/kernelci-deploy/./kernel.py", line 43, in main
repo = kernelci.GITHUB.get_repo(repo_name)
File "/usr/lib/python3/dist-packages/github/MainClass.py", line 294, in get_repo
headers, data = self.__requester.requestJsonAndCheck(
File "/usr/lib/python3/dist-packages/github/Requester.py", line 275, in requestJsonAndCheck
return self.__check(*self.requestJson(verb, url, parameters, headers, input, self.__customConnection(url)))
File "/usr/lib/python3/dist-packages/github/Requester.py", line 286, in __check
raise self.__createException(status, responseHeaders, output)
github.GithubException.RateLimitExceededException: 403 {'message': "API rate limit exceeded for 52.250.7.52. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)", 'documentation_url': 'https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting'}
```
So the solution would seem to be about using authenticated requests.