[Release 1.28] Fix bug when using tailscale config by file

[manuelbuil]

Backport: #10074
Issue: #10141

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 0.00%. Comparing base (f5dc8e7) to head (8c79807).

Files	Patch %	Lines
pkg/cli/agent/agent.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           release-1.28   #10144       +/-   ##
=================================================
- Coverage         42.97%    0.00%   -42.98%     
=================================================
  Files               158      155        -3     
  Lines             16414    13885     -2529     
=================================================
- Hits               7054        0     -7054     
- Misses             8117    13885     +5768     
+ Partials           1243        0     -1243     

Flag	Coverage Δ
e2etests	0.00% <0.00%> (-46.18%)	⬇️
unittests	?

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