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 cucumber_expressions/cucumber_expressions/go.sum (see #17709743808 logs) rather than cucumber_expressions/go/go.sum.

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

Metric	Previous	Now	Delta	% Change
Total Workflow Runner Time	1m 33s (99s)	1m 19s (79s)	-20s	-20.2%
Fastest Job	18s	17s	-1s	-5.6%
Average Job	31s	26.3s	-4.7s	-15.2%

• 20.2% reduction in runner uptime - saving $40.54 and 715 runner minutes compared to 3.5 days total runner usage for this workflow over the past year (assuming GitHub Actions pricing)
• 15.2% faster average job execution - continuous feedback in 17s

🏷️ 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