pkgsi686Linux.nixosTests.gitlab: fix 32 bit tests

GitLab 11.5.1 dropped the dependency to posix_spawn, which is broken on
32bit. (See https://gitlab.com/gitlab-org/gitlab-ce/issues/53525)

The only part missing is decreasing virtualisation.memorySize to
something that a 32 bit qemu still executes.

The maximum seems to be 2047, and tests passed with that value for me.