Add Ubuntu 20.04, `s/-j3/-j$(nproc)`

.github/workflows/build.yml

@@ -5,7 +5,8 @@ on: [push, pull_request]
 jobs:
   build_qt:
     name: With QT GUI
-    runs-on: ubuntu-18.04
+    runs-on: ["ubuntu-18.04", "ubuntu-20.04"]
+
     steps:
     - uses: actions/checkout@v2
       with:
@@ -18,4 +19,4 @@ jobs:
     - name: build application
       run: |
         qmake
-        make -j3
+        make -j$(nproc)