nixos/sddm: delete QML cache

Prior to Qt 5.9.2, there is a QML cache invalidation bug which causes SDDM to
segfault when upgrading Plasma.

See also: https://bugreports.qt.io/browse/QTBUG-62302

nixos/modules/services/x11/display-managers/sddm.nix

@@ -19,6 +19,17 @@ let
 
   Xsetup = pkgs.writeScript "Xsetup" ''
     #!/bin/sh
+
+    # Prior to Qt 5.9.2, there is a QML cache invalidation bug which sometimes
+    # strikes new Plasma 5 releases. If the QML cache is not invalidated, SDDM
+    # will segfault without explanation. We really tore our hair out for awhile
+    # before finding the bug:
+    # https://bugreports.qt.io/browse/QTBUG-62302
+    # We work around the problem by deleting the QML cache before startup. It
+    # will be regenerated, causing a small but perceptible delay when SDDM
+    # starts.
+    rm -fr /var/lib/sddm/.cache/sddm-greeter/qmlcache
+
     ${cfg.setupScript}
   '';