nixos/tests/gdk-pixbuf: fix test on i686 (#46916)

Test didn't run because it tried to create a VM with 4096M RAM
but qemu-system-i386 has a hard 2047M memory limit.
- reduce memory to 2047M on i686.
- increase timeout 300s -> 1800s because the tests are much slower
  on i686 and timed out.