ci: run mainnet spectests on 4core

Author: spiral-ladder

.github/workflows/CI.yml

@@ -243,7 +243,7 @@ jobs:
   spec-test-mainnet:
     name: spec tests - mainnet
     # It takes time to download and run spec tests, so we only run them on ubuntu-latest
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-latest-4core
     needs: build-test
     steps:
       - name: Checkout Repository