Fix export template when using dummy links

[hellt]

fix #2480
when dummy links are used, the endpoint section in the export template will now contain only "a" side, with the peer set to dummy:

  "links": [
    {
      "a": {
        "node": "l1",
        "interface": "eth1",
        "mac": "aa:c1:ab:97:93:32",
        "peer": "dummy"
      }
    }
  ]
}

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 52.43%. Comparing base (43367b9) to head (9ff42b3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
clab/export.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2483      +/-   ##
==========================================
- Coverage   52.52%   52.43%   -0.10%     
==========================================
  Files         172      172              
  Lines       17692    17692              
==========================================
- Hits         9292     9276      -16     
- Misses       7422     7439      +17     
+ Partials      978      977       -1     

Files with missing lines	Coverage Δ
clab/export.go	40.57% <0.00%> (-31.89%)	⬇️

... and 1 file with indirect coverage changes