Fix unit test workflow: exclude cmd package with no tests

The cmd/selfhost_s3 package has no test files, which causes
Go's coverage tool to fail with "no such tool covdata" error.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: pierre-b

.github/workflows/go.yml

@@ -26,8 +26,8 @@ jobs:
 
       - name: Run unit tests
         run: |
-          # Run unit tests (exclude integration tests)
-          go test -race -coverprofile=coverage.txt -covermode=atomic $(go list ./... | grep -v '/integration') -v
+          # Run unit tests (exclude integration tests and cmd which has no tests)
+          go test -race -coverprofile=coverage.txt -covermode=atomic $(go list ./... | grep -v '/integration' | grep -v '/cmd/') -v
 
       - name: Upload coverage to Codecov
         uses: codecov/codecov-action@v5