Investigate CI flakiness for repo-updater/repos/sources_test.go

Created by: mrnugget

We had to run our build two times to make it pass on CI: https://buildkite.com/sourcegraph/sourcegraph/builds/34892#0d29f393-5cd6-4406-84f5-2723c5d47752

FAIL: TestGithubSource_GetRepo (0.02s) | 54s
-- | --
  | [2019-05-14T14:31:11Z]     --- FAIL: TestGithubSource_GetRepo/GITHUB-DOT-COM/not_found (0.00s)
  | [2019-05-14T14:31:11Z]         sources_test.go:857: error:
  | [2019-05-14T14:31:11Z]             have: "GitHub repository not found"
  | [2019-05-14T14:31:11Z]             want: "request to http://github-proxy/repos/foobarfoobarfoobar/please-let-this-not-exist returned status 404: Not Found"
  | [2019-05-14T14:31:11Z] FAIL

The same test didn't fail in previous PR and it doesn't fail locally. Our best guess at the moment is that this has something to do with the caching logic in the Github client:

https://github.com/sourcegraph/sourcegraph/blob/d6294814e1102a871eb28669f960997434904baf/pkg/extsvc/github/repos.go#L139-L145

This is also related to #3895 (closed)