ci: use free github runners for pr builds and add renovate-check

Author: rajannpatel

.github/workflows/build.yml

@@ -12,3 +12,5 @@ concurrency:
 jobs:
   build:
     uses: canonical/sdkcraft-actions/.github/workflows/build.yml@main
+    with:
+      runs-on: '["ubuntu-24.04"]'

.github/workflows/renovate-check.yml

@@ -0,0 +1,10 @@
+name: Renovate check
+
+on:
+  pull_request:
+    paths:
+      - renovate.json
+
+jobs:
+  renovate-check:
+    uses: canonical/sdkcraft-actions/.github/workflows/renovate-check.yml@main