Flush matnwb tarball before untarring

[jwodder]

While testing #28, I kept running into intermittent errors with tar complaining about an early EOF. This appears to fix the problem.

[codecov[bot]]

Codecov Report

Base: 63.70% // Head: 63.54% // Decreases project coverage by -0.17% :warning:

Coverage data is based on head (8f4e800) compared to base (76840d5). Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #31 +/- ## ========================================== - Coverage 63.70% 63.54% -0.17% ========================================== Files 6 6 Lines 383 384 +1 Branches 99 99 ========================================== Hits 244 244 - Misses 122 123 +1 Partials 17 17 ``` | [Impacted Files](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [code/src/healthstatus/\_\_main\_\_.py](https://codecov.io/gh/dandi/dandisets-healthstatus/pull/31?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-Y29kZS9zcmMvaGVhbHRoc3RhdHVzL19fbWFpbl9fLnB5) | `61.79% <0.00%> (-0.71%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi)

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[yarikoptic]

interestingly odd, thanks.