[Release 1.29] Fix bug when using tailscale config by file

[manuelbuil]

Backport: #10074
Issue: #10139

[codecov]

Codecov Report

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

Project coverage is 24.90%. Comparing base (4e53a32) to head (d64d50d).

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

Additional details and impacted files

@@                Coverage Diff                @@
##           release-1.29   #10142       +/-   ##
=================================================
- Coverage         50.12%   24.90%   -25.22%     
=================================================
  Files               158      158               
  Lines             14038    14038               
=================================================
- Hits               7036     3496     -3540     
- Misses             5671     9851     +4180     
+ Partials           1331      691      -640     

Flag	Coverage Δ
e2etests	4.88% <0.00%> (-41.80%)	⬇️
inttests	18.98% <0.00%> (-18.16%)	⬇️
unittests	16.57% <ø> (ø)

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