Validate output of MIRI is successful on CI

[alamb]

Closes #581

Rationale for this change

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)

What changes are included in this PR?

Check output of MIRI return

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #578 (2049c28) into master (68a4b5b) will not change coverage.
The diff coverage is n/a.

❗ Current head 2049c28 differs from pull request most recent head adc189d. Consider uploading reports for the commit adc189d to get more accurate results

@@           Coverage Diff           @@
##           master     #578   +/-   ##
=======================================
  Coverage   82.46%   82.46%           
=======================================
  Files         167      167           
  Lines       46205    46205           
=======================================
  Hits        38101    38101           
  Misses       8104     8104           

Impacted Files	Coverage Δ
arrow/src/array/ffi.rs	100.00% <ø> (ø)
arrow/src/compute/kernels/sort.rs	94.15% <ø> (ø)
arrow/src/datatypes/ffi.rs	72.86% <ø> (ø)
arrow/src/ffi.rs	87.07% <ø> (ø)

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[alamb]

FYI @jorgecarleitao @nevi-me and @roee88 -- I am merging this PR in so that we do not allow MIRI failures in again on CI 🎉