Fix Makefile parsing error with multi-path GOPATH

[clcollins]

Summary

Fixes Make parsing error when GOPATH contains multiple colon-separated paths (common with gvm pkgsets).

The Makefile now extracts only the first component of GOPATH, which:

• Avoids Make interpreting colons as pattern rule delimiters
• Points to the correct location where go install puts binaries
• Works with both single-path and multi-path GOPATH configurations

Test plan

• Verified make build works with multi-path GOPATH
• Verified make lint target parses correctly
• Tested with single-path GOPATH (still works)

🤖 Generated with Claude Code

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: clcollins

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.63%. Comparing base (84bd137) to head (d7e3dcf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #123   +/-   ##
=======================================
  Coverage   62.63%   62.63%           
=======================================
  Files           6        6           
  Lines         736      736           
=======================================
  Hits          461      461           
  Misses        233      233           
  Partials       42       42           

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