Update build.yml

.github/workflows/build.yml

@@ -12,6 +12,8 @@ jobs:
     runs-on: ubuntu-latest
     container: docker.io/library/archlinux:latest
     steps:
+    - name: Checkout Source
+      uses: actions/checkout@v2
     - name: dependencies
       run: pacman -Sy; pacman -S --noconfirm fishui qt5-svg extra-cmake-modules qt5-tools
     - name: Build