ci: migrate from archived useblacksmith/cache to actions/cache@v4

Blacksmith archived useblacksmith/cache on 2025-10-14 — their runners
now transparently intercept upstream actions/cache@v4 and route it to
the same fast colocated backend. Same performance, actively maintained,
no Node.js 20 deprecation warning.

https://github.com/useblacksmith/cache (archived)
https://www.blacksmith.sh/blog/cache (migration rationale)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vvillait88

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
     steps:
       - uses: useblacksmith/checkout@v1
       - uses: astral-sh/setup-uv@v7
-      - uses: useblacksmith/cache@v1
+      - uses: actions/cache@v4
         with:
           path: |
             ~/.cache/uv