Fix bug when using tailscale config by file

[manuelbuil]

Proposed Changes

When passing the config of tailscale using the config file, Tailscale was not working correctly in the agent. The reason is that we were setting the variable cmds.AgentConfig.VPNAuth which is not used by the agent. A few lines before, k3s is doing cfg := cmds.AgentConfig and after that, anything set in cmds.AgentConfig will get lost. Therefore, we should set the variable cfg.VPNAuth instead, which is the one being read by the agent. This PR does exactly that

Types of Changes

Bugfix

Verification

Deploy k3s+tailscale using the docs and using the vpn-auth-file config

Testing

Linked Issues

#10064

User-Facing Change

Fix bug when using `vpn-auth-file` in the agent

Further Comments

[codecov]

Codecov Report

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

Project coverage is 37.00%. Comparing base (fe7d114) to head (be94efe).
Report is 3 commits behind head on master.

Files	Patch %	Lines
pkg/cli/agent/agent.go	0.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #10074       +/-   ##
===========================================
- Coverage   48.92%   37.00%   -11.93%     
===========================================
  Files         158      156        -2     
  Lines       14032    13999       -33     
===========================================
- Hits         6865     5180     -1685     
- Misses       5885     7770     +1885     
+ Partials     1282     1049      -233     

Flag	Coverage Δ
e2etests	36.10% <0.00%> (-10.68%)	⬇️
inttests	6.10% <0.00%> (?)
unittests	?

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