Enter ./cpp first before CI tasks, take 2

.github/workflows/main.yaml

@@ -5,6 +5,9 @@ on: [push, pull_request]
 jobs:
   "test_compile_cpp":
     runs-on: ubuntu-18.04
+    defaults:
+      run:
+        working-directory: ./cpp
     steps:
       - uses: actions/checkout@v2
 
@@ -15,9 +18,6 @@ jobs:
           path: compilers
           key: ${{ runner.os }}-${{ hashFiles('./cpp/ci-prepare-bionic.sh') }}
 
-      - name: Prepare compilers
-        run: cd ./cpp
-
       - name: Prepare compilers
         if: steps.prepare-compilers.outputs.cache-hit != 'true'
         run: source ./ci-prepare-bionic.sh ./compilers SETUP true || true