nixos/tests/virtualbox: Fix @shell@ expansion

This has surfaced since f803270b

.

The commit bumped bash to version 4.4, which caused to change the order
of --subst-var flags in substituteAll, which this test was relying on,
because it added a @shell@ to boot.initrd.postMountCommands.

Our substituter is currently working a bit like this:

original.replace('@var1@', 'val1').replace('@var2@', 'val2')...

Unfortunately, this means that if @var2@ occurs within @var1@ it is
replaced by the new value, so the order of the substvars actually
matter. I highly doubt that we want a behaviour like this and I'm
wondering why it didn't occur to me as a problem while writing the
initial implementation of the VirtualBox tests.

Whether to get rid of this and disallowing substitution of substvars
within substvars is another topic which I think needs discussion in a
different place.

As for now, I'm using stdenv.shell, because the closure size of this
should fit within the initrd, so it's fine especially because it's just
a test.

Tested with the net-hostonlyif and systemd-detect-virt tests and they
both succeed with this change.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: @globin on IRC