ci: let govulncheck pick the toolchain it needs

golang.org/x/vuln v1.8.0 requires Go 1.26 while main's Go pipeline pins 1.25
with GOTOOLCHAIN=local, so installing govulncheck@latest fails on every Go leg.
Allowing the toolchain switch for that one step unblocks the build-go jobs
this PR triggers; #109 moves the pipeline to 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

@@ -68,6 +68,8 @@ 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