ci: start cache building immediately without waiting for format checks

[huth-stacks]

What

Start the nextest archive build immediately when CI triggers, instead of waiting for rustfmt and changelog checks to pass first.

Why

The create-cache job depends on rustfmt, changelog-check, and check-release — three trivial jobs (~28 seconds total). This forces the 15-minute nextest archive build to wait for formatting validation, adding 28 seconds to the critical path for no benefit. All test workflows independently depend on both create-cache AND the format checks, so a format failure still blocks test results.

The Change

In .github/workflows/ci.yml, remove the needs block from create-cache:

-    needs:
-      - rustfmt
-      - changelog-check
-      - check-release

4 lines deleted in 1 file.

How to Verify

• create-cache should appear at Stage 0 in the workflow graph (parallel with rustfmt)
• All test workflows should still wait for both create-cache and rustfmt before running

Metrics to Track

Metric	Baseline	Expected
create-cache start delay	~28s after trigger	~0s (immediate)

Security Checklist

• No new permissions granted
• No secrets exposure
• Cache isolation unchanged — same content, just built earlier

Part of CI Optimization Series

PR 2 of 7. Expected impact: ~28s off critical path.