ci: drop the stale Go pipeline tweak now that main ships Go 1.27

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FHtZ26sF1zNNrEh1wDGzBa

Author: claude

.github/workflows/build-go-pipeline.yaml

@@ -90,8 +90,6 @@ jobs:
 
       - name: Install govulncheck
         run: go install golang.org/x/vuln/cmd/govulncheck@latest
-        env:
-          GOTOOLCHAIN: auto
 
       - name: Run dependency vulnerability scan
         continue-on-error: true