mg-repro-setup: Make ordinary alloc: work again

mg-repro-setup: Make ordinary alloc: work again

In e7febe5f6edc, we hosted an error earlier in the script.  But in the
old location, OSSTEST_TASK would always be set if statictask was.  In
the new one, that hasn't been done yet.

No release implications since it touches a by-hand utility only.

Fixes: e7febe5f6edc "mg-repro-setup: Promote an error test to before builds (nfc)"
CC: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>