nixos/release: Test vbox on x86_64-linux only.

The i686-linux test has never worked and I wrote the VM test only on
x86_64-linux to verify whether hardening mode works. I don't know why it
fails on i686-linux, but that might be because the inner VirtualBox VM
we're starting during the VM test doesn't use hardware virtualization.

Closes #5708.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>