manibuilder: Let build tests fail with the original error code

We can still run all tests with `make -k`.

Change-Id: Ib59b8863186e5f7ecbaa2aa65cdbd51913dcf8e7
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/70913
Tested-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
2 files changed