gastown: pick free port for test

[stefanb]

https://github.com/Homebrew/homebrew-core/actions/runs/27825740996/job/82353326998?pr=288803#step:3:11779

  ==> /opt/homebrew/Cellar/gastown/1.1.0/bin/gt install
  Error: Dolt port 3307 is already in use
  Port is held by PID 24998
  
  Another Gas Town instance is using this port. Specify a free port:
  
    gt install --dolt-port 3308

Ref:

• go 1.27rc2, go@1.26 1.26.5 (new formula) #288803

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