Fix CI (#2028)

.github/workflows/build.yml

@@ -29,13 +29,19 @@ jobs:
         uses: actions/cache@v2
         with:
           path: ../Qt
-          key: ${{ runner.os }}-QtCache-v2
+          key: ${{ runner.os }}-QtCache-v5
+
+      # LINUX
+      - name: Install p7zip (Ubuntu)
+        if: startsWith(matrix.os, 'ubuntu')
+        run: sudo apt-get update && sudo apt-get -y install p7zip-full
 
       - name: Install Qt
         uses: jurplel/install-qt-action@v2
         with:
           mirror: 'http://mirrors.ocf.berkeley.edu/qt/'
           cached: ${{ steps.cache-qt.outputs.cache-hit }}
+          extra: --external 7z
 
       # WINDOWS
       - name: Cache conan