make-flight: Keep using buster for L2 guest in nested tests

make-flight: Keep using buster for L2 guest in nested tests

When starting the installation of the L2 guest, L0 kills L1. Switching
the L2 guest back to Debian Buster works fine, so do that to prevent
regression in the test.

Part of the logs from the host L0:

> domain_crash called from arch/x86/hvm/vmx/vvmx.c:2770
> Domain 3 (vcpu#0) crashed on cpu#4:
> d3v0 vmentry failure (reason 0x80000021): Invalid guest state (2)

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>