enhance(client): use scm token as secondary auth header for exec client

[ecrupper]

Follow up from go-vela/server#1337 and go-vela/sdk-go#369

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.16%. Comparing base (14f88b0) to head (7d2bbdf).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/vela-worker/client.go	0.00%	10 Missing ⚠️
cmd/vela-worker/exec.go	0.00%	4 Missing ⚠️

❌ Your project status has failed because the head coverage (58.16%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #657      +/-   ##
==========================================
- Coverage   58.24%   58.16%   -0.08%     
==========================================
  Files         123      123              
  Lines        8368     8379      +11     
==========================================
  Hits         4874     4874              
- Misses       3283     3294      +11     
  Partials      211      211              

Files with missing lines	Coverage Δ
cmd/vela-worker/exec.go	0.00% <0.00%> (ø)
cmd/vela-worker/client.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.