Ensure output of MIRI is checked for success

[alamb]

Describe the bug
MIRI tests are run on each CI check but the CI will pass even if the MIRI checks fail

To Reproduce
Introduce a MIRI change and observe CI still passes

Expected behavior
If MIRI fails, CI tests should also fail

Additional context
We started running MIRI again in #421, but while discussing with @jorgecarleitao
on jorgecarleitao/arrow2#209, it turned out that we were still ||true ing the output (aka not validating that MIRA was running cleanly)