Use a mirror for Qt downloads

Source is broken

Author: cmaughan

.github/workflows/build.yml

@@ -74,6 +74,7 @@ jobs:
     - name: Install Qt
       uses: jurplel/install-qt-action@v2
       with:
+        mirror: 'http://mirrors.ocf.berkeley.edu/qt/'
         cached: ${{ steps.cache-qt2.outputs.cache-hit }}
 
       # Latest Erlang on Ubuntu