Change to dart pub instead of pub

Author: tmthecoder

.github/workflows/dargon2_tests.yml

@@ -19,6 +19,6 @@ jobs:
     - uses: actions/checkout@v2
     - uses: dart-lang/setup-dart@v1
     - name: Install dependencies
-      run: pub get
+      run: dart pub get
     - name: Run tests
-      run: pub run test
+      run: dart pub run test