QT is installed only in a windows native_mixed job

kiwix-desktop is built under Windows only in native_mixed config and QT
is not required for anything else. Therefore it doesn't make sense to
run QT installation in Windows native_dyn and native_static CI/CD jobs.
That will reduce the risk of those jobs failing because of problems with
QT mirrors.

.github/workflows/cd.yml

@@ -37,6 +37,7 @@ jobs:
         pip3 install meson pytest requests distro paramiko
         pip3 install --no-deps $GITHUB_WORKSPACE
     - name: Install QT
+      if: ${{ matrix.config == 'native_mixed' }}
       uses: jurplel/install-qt-action@v4
       with:
         version: 5.15.2

.github/workflows/ci.yml

@@ -35,6 +35,7 @@ jobs:
         pip3 install meson pytest requests distro paramiko
         pip3 install --no-deps $GITHUB_WORKSPACE
     - name: Install QT
+      if: ${{ matrix.config == 'native_mixed' }}
       uses: jurplel/install-qt-action@v4
       with:
         version: 6.4.3