ci: use slim Ubuntu runners (#256)

It will be more environmentally friendly and cheaper (at least for GitHub, because it's a public project and GitHub pays for it, not you)

GitHub blog post about new 1 vCPU runners: https://github.blog/changelog/2025-10-28-1-vcpu-linux-runner-now-available-in-github-actions-in-public-preview

Pricing (tl;dr: $0.002/min): https://docs.github.com/en/billing/reference/actions-runner-pricing

GitHub-hosted runners reference: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#standard-github-hosted-runners-for--private-repositories

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ permissions:
 
 jobs:
   ci:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     timeout-minutes: 10
     steps:
       - name: Checkout