TEMP: force setup-uv attempt-1 failure to validate the retry path

Points the test job's first attempt at an unreachable manifest URL so
all 20 legs exercise the retry step. Will be dropped from this branch.

No-Verification-Needed: temporary validation scaffolding, dropped before merge

Author: maxisbey

.github/workflows/shared.yml

@@ -82,6 +82,7 @@ jobs:
         with:
           enable-cache: true
           version: 0.9.5
+          manifest-file: https://example.invalid/uv.ndjson
 
       - name: Install uv (retry)
         if: steps.setup-uv.outcome == 'failure'