cache Qt installation in Windows CI jobs (#3769)

* CI-windows.yml: cache Qt installation

* release-windows.yml: store Qt version in env

* release-windows.yml: cache Qt installation

.github/workflows/CI-windows.yml

@@ -91,6 +91,14 @@ jobs:
           7z x z3-%Z3_VERSION%-${{ matrix.arch }}-win.zip -oexternals -r -y || exit /b !errorlevel!
           move externals\z3-%Z3_VERSION%-${{ matrix.arch }}-win externals\z3 || exit /b !errorlevel!
 
+      - name: Cache Qt ${{ matrix.qt_ver }}
+        if: matrix.qt_ver != '' && matrix.arch == 'x64'
+        id: cache-qt
+        uses: actions/cache@v1  # not v2!
+        with:
+          path: ../Qt
+          key: Windows-QtCache-${{ matrix.qt_ver }}-qtcharts
+
       # no 32-bit Qt available
       - name: Install Qt ${{ matrix.qt_ver }}
         if: matrix.qt_ver != '' && matrix.arch == 'x64'
@@ -98,6 +106,7 @@ jobs:
         with:
           version: ${{ matrix.qt_ver }}
           modules: 'qtcharts'
+          cached: ${{ steps.cache-qt.outputs.cache-hit }}
 
       - name: Install missing Python packages
         if: matrix.qt_ver == ''

.github/workflows/release-windows.yml

@@ -27,6 +27,7 @@ jobs:
       PCRE_VERSION: 8.45
       # see https://github.com/Z3Prover/z3/releases:
       Z3_VERSION: 4.8.10
+      QT_VERSION: 5.15.2
 
     steps:
       - uses: actions/checkout@v2
@@ -76,11 +77,19 @@ jobs:
           7z x z3-%Z3_VERSION%-x64-win.zip -oexternals -r -y
           move externals\z3-%Z3_VERSION%-x64-win externals\z3
 
-      - name: Install Qt
+      - name: Cache Qt ${{ env.QT_VERSION }}
+        id: cache-qt
+        uses: actions/cache@v1  # not v2!
+        with:
+          path: ../Qt
+          key: Windows-QtCache-${{ env.QT_VERSION }}-qtcharts-qthelp
+
+      - name: Install Qt ${{ env.QT_VERSION }}
         uses: jurplel/install-qt-action@v2
         with:
-          version: '5.15.2'
+          version: ${{ env.QT_VERSION }}
           modules: 'qtcharts qthelp'
+          cached: ${{ steps.cache-qt.outputs.cache-hit }}
 
       - name: Create .qm
         run: |