colord: fix use of /var

Currently the package is built with /var in $out/var. That fails when it
tries to create/write things at runtime (nix store is read-only).
Instead, tell it to use /var (global directory) and fixup the
installation phase so it doesn't touch /var (leave that for runtime).

This unbreaks the colord dbus service, which apparently is needed by
cups to create color profiles for printers.

pkgs/tools/misc/colord/default.nix

···
   configureFlags = [
     "--with-udevrulesdir=$out/lib/udev/rules.d"
     "--with-systemdsystemunitdir=$(out)/etc/systemd/system"
+    "--localstatedir=/var"
     "--disable-bash-completion"
   ];
 
+  # don't touch /var at install time, colord creates what it needs at runtime
+  postPatch = ''
+    sed -i -e "s|if test -w .*;|if false;|" src/Makefile.in
+    sed -i -e "s|if test -w .*;|if false;|" src/Makefile.am
+  '';
+
   buildInputs = [ glib polkit pkgconfig intltool gusb libusb1 lcms2 sqlite systemd dbus gobjectIntrospection
                   bashCompletion argyllcms automake autoconf libgudev ];
 
   postInstall = ''
-    rm -fr $out/var/lib/colord
     mkdir -p $out/etc/bash_completion.d
     cp -v data/colormgr $out/etc/bash_completion.d
   '';