enhance(auth): add install token as possible auth method

[ecrupper]

Will look for VELA_GIT_TOKEN in environment (where the install token is set during a build). Still preferring the VELA_TOKEN value if supplied so this is not breaking.

[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.34%. Comparing base (15bda2c) to head (d79e521).

Files with missing lines	Patch %	Lines
cmd/vela-cli/main.go	0.00%	5 Missing ⚠️
internal/client/client.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
- Coverage   71.44%   71.34%   -0.10%     
==========================================
  Files         174      174              
  Lines        4718     4726       +8     
==========================================
+ Hits         3371     3372       +1     
- Misses       1042     1048       +6     
- Partials      305      306       +1     

Files with missing lines	Coverage Δ
internal/client/validate.go	80.00% <100.00%> (ø)
internal/client/client.go	76.47% <50.00%> (-4.18%)	⬇️
cmd/vela-cli/main.go	0.00% <0.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.