.github/workflows: migrate to Blacksmith runners

Switch from ubuntu-latest to blacksmith-4vcpu-ubuntu-2404 for faster
CI builds with better caching.

Matches evalops/llm-gateway#94.

Author: haasonsaas

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: blacksmith-4vcpu-ubuntu-2404
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5