Ensure a successful MIRI Run on CI

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-11715

Now we have the MIRI check setup to pass even of cargo miri returns an error.

https://github.com/apache/arrow/blob/master/.github/workflows/rust.yml#L263-L264

          # Ignore MIRI errors until we can get a clean run
          cargo miri test || true

Goal is to make MIRI pass and then remove this check in CI

[jhorstmann]

@alamb I think this was fixed by #421 and can be closed

[alamb]

Agree -- thanks @jhorstmann !