fix: go GitHub workflow cache

[kieran-ryan]

🤔 What's changed?

Explicit go lock file path to enable caching in GitHub workflow.

⚡️ What's your motivation?

Improve CI performance.

Addressing that the setup-go action expects the lock file at messages/message/go.sum (see #50333999175 logs) rather than messages/go/go.sum.

Restore cache failed: Dependencies file is not found in /Users/runner/work/messages/messages. Supported file pattern: go.sum

Metric	Previous	Now	Delta	% Change
Total Workflow Runner Time	2m 13s (133s)	1m 34s (94s)	-39s	-29.3%
Fastest Job	24s	12s	-12s	-50%
Average Job	33.3s	23.5s	-9.8s	-29.4%

• 29.3% reduction in runner uptime - saving $30.82 and 1799 runner minutes compared to 4.3 days total runner usage for this workflow over the past year (assuming GitHub Actions pricing)
• 29.4% faster average job execution - continuous feedback in 12s

🏷️ What kind of change is this?

• 🐛 Bug fix (non-breaking change which fixes a defect)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5cf3d14) to head (d0ce69e).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #334   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          200       200           
=========================================
  Hits           200       200           

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