Update main.yml

update action spec to be able to run on github provided runner

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: Testing project
 on: push
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - name: Checkout repo
       uses: actions/checkout@v2