nixos/tests/installer: don't break under i686

Currently, the installer tests just hang after the initial install phase
on i686 because qemu just quits because of the gic parameter.

Fix this by doing x86 things for both x86-64 and i686.

nixos/tests/installer.nix

@@ -75,7 +75,7 @@ let
     else ''
       def assemble_qemu_flags():
           flags = "-cpu max"
-          ${if system == "x86_64-linux"
+          ${if (system == "x86_64-linux" || system == "i686-linux")
             then ''flags += " -m 1024"''
             else ''flags += " -m 768 -enable-kvm -machine virt,gic-version=host"''
           }