CI: Download Qt5 conditionally on Linux

Qt6 libraries are installed if the package is available on the system.
Uses the inverted condition to install Qt5 libraries to avoid
unnecessarily install both libraries.

CI/linux/01_install_dependencies.sh

@@ -37,7 +37,10 @@ install_qt5-deps() {
     status "Install Qt5 dependencies"
     trap "caught_error 'install_qt5-deps'" ERR
 
-    sudo apt-get install --no-install-recommends -y $@
+    _QT6_AVAILABLE="$(sudo apt-cache madison qt6-base-dev)"
+    if [ -z "${_QT6_AVAILABLE}" ]; then
+        sudo apt-get install --no-install-recommends -y $@
+    fi
 }
 
 install_qt6-deps() {