Github CI: Add qt6

Author: cblp

.github/workflows/haskell.yaml

@@ -17,7 +17,7 @@ jobs:
     steps:
 
     - name: Install extra software
-      run: sudo apt-get install -y colordiff
+      run: sudo apt-get install -y colordiff qt6-base-dev
 
     - uses: actions/checkout@v2