Fix Go linting issues: replace WriteString(fmt.Sprintf) with fmt.Fprintf

[Copilot]

Fixed staticcheck linting violations in artifact manager test utilities.

Changes

• QF1012: Replaced 13 instances of sb.WriteString(fmt.Sprintf(...)) with fmt.Fprintf(&sb, ...) for more efficient string building
• S1009: Removed redundant nil check before len() call on map (len of nil map is defined as 0)

All changes were in pkg/workflow/artifact_manager_workflows_test.go helper functions that generate markdown summaries.

// Before
sb.WriteString(fmt.Sprintf("### Job: `%s`\n\n", jobName))

// After
fmt.Fprintf(&sb, "### Job: `%s`\n\n", jobName)

This eliminates unnecessary string allocations while maintaining identical output.

Original prompt

Lint go

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.