udisk2: add sysconfdir flags, fixes changing options in gnome-disk-utility

pkgs/os-specific/linux/udisks/2-default.nix

@@ -64,6 +64,7 @@ stdenv.mkDerivation rec {
 
   configureFlags = [
     (lib.enableFeature (stdenv.buildPlatform == stdenv.hostPlatform) "gtk-doc")
+    "--sysconfdir=/etc"
     "--localstatedir=/var"
     "--with-systemdsystemunitdir=$(out)/etc/systemd/system"
     "--with-udevdir=$(out)/lib/udev"
@@ -75,6 +76,10 @@ stdenv.mkDerivation rec {
     "INTROSPECTION_TYPELIBDIR=$(out)/lib/girepository-1.0"
   ];
 
+  installFlags = [
+    "sysconfdir=${placeholder "out"}/etc"
+  ];
+
   enableParallelBuilding = true;
 
   doCheck = true;