Pre-install Qrca flatpak

It's required for "connect to Wi-Fi via QR code" functionality just
merged into plasma-nm, and explicitly recommended in
https://community.kde.org/Distributions/Packaging_Recommendations

mkosi.extra/usr/lib/install-flatpaks

@@ -4,5 +4,5 @@
 
 set -eux
 
-flatpak install --noninteractive --assumeyes --sideload-repo=/usr/share/factory/var/lib/flatpak/.ostree/repo org.gtk.Gtk3theme.Breeze org.kde.gwenview org.kde.okular org.kde.kate org.kde.kwrite org.kde.haruna org.kde.ark org.mozilla.firefox
+flatpak install --noninteractive --assumeyes --sideload-repo=/usr/share/factory/var/lib/flatpak/.ostree/repo org.gtk.Gtk3theme.Breeze org.kde.gwenview org.kde.okular org.kde.kate org.kde.kwrite org.kde.haruna org.kde.qrca org.kde.ark org.mozilla.firefox
 flatpak repair # Get localizations back now

mkosi.postinst.chroot

@@ -77,6 +77,7 @@ download_flatpaks() {
         kate
         haruna
         okular
+        qrca
     )
 
     # Add Nightly repos