Fix: Check for git authentication when GITHUB_TOKEN exists

[jsong336]

TL;DR

When flytectl demo start is ran with GITHUB_TOKEN, it throws 401 error. Setting GITHUB_TOKEN empty works.
flyteorg/flyte#4879

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

• Validating GITHUB client before creating gh

Tracking Issue

https://github.com/flyteorg/flyte/issues/

Follow-up issue

[pingsutw]

LGTM, thank you

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (9c87165) 67.69% compared to head (af6308b) 67.65%.

Files	Patch %	Lines
pkg/github/githubutil.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #463      +/-   ##
==========================================
- Coverage   67.69%   67.65%   -0.05%     
==========================================
  Files         148      148              
  Lines        6639     6643       +4     
==========================================
  Hits         4494     4494              
- Misses       1855     1859       +4     
  Partials      290      290              

Flag	Coverage Δ
unittests	67.65% <0.00%> (-0.05%)	⬇️

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[jsong336]

@pingsutw hmmm not sure what's best way to write test for this, are we able bypass test for this PR?