gha: credential-less access

Author: csegarragonz

.github/workflows/tests.yml

@@ -18,9 +18,6 @@ jobs:
     runs-on: ubuntu-latest
     container:
       image: faasm.azurecr.io/cpp-sysroot:0.5.0
-      credentials:
-        username: ${{ secrets.ACR_SERVICE_PRINCIPAL_ID }}
-        password: ${{ secrets.ACR_SERVICE_PRINCIPAL_PASSWORD }}
     steps:
       # --- Update code ---
       - name: "Checkout code"