autobuild: Silence shellcheck's warning about SC2031

shellcheck is confused about $MAKEFLAGS's scoping when it scans the
entire SPDK repo. It thinks that _build_release() may potentially
change it, and since it's defined as a subshell function, this change
may be not visible in a global scope. This is true in theory, but in
practice this is not the case at all since _build_release() simply
passes $MAKEFLAGS along to make.

In per-patch, shellcheck validates only scripts for which changes are
being applied in given patch, hence this issue is not seen there - it
wasn't seen while validating 58674af0 since that patch didn't touch
autobuild.sh at all. It's done this way mainly for the sake of
performance.

In vs-dpdk, however, there's still old autobuild-vg (scheduled for
migration) job in use which scans entire repo, hence tripping over
the SC2031 directive.

Since this is a false positive, simply disable it.

Change-Id: Ic7e58a1a63f88e2a9b994dc510aa44216c28d3a9
Signed-off-by: Michal Berger <michal.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/24584


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <jim.harris@samsung.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>