Fix issue with private repositories not being indexed

[brendan-kellam]

Issue template for later::

Problem

When running zoekt-mirror-github with the -user flag specified, private repositories are not indexed even when a valid .github-token file is present.

Repro steps:

1. Create ~/.github-token with a valid PAT token
2. Run the following command:
   zoekt-mirror-github -dest data -delete -user userName
3. Observe the repositories indexed

Expected:

All repositories (both public and private) associated with the user should be indexed.

Actual:

Only public repositories are indexed

How this PR fixes it:
By using a empty string for the user, the go github library will fallback on using the users/repos endpoint instead of the users/{user-name}/repos endpoint. For whatever reason, the latter does not return private repositories even when a authentication token is provided. The former does. Thus, this is a quick n' dirty fix that gets around this issue.

From the github library: