test: Fix test failure due to .coverage changing during tar creation.

[smoser]

This seems to be a transient failure, but is legitimate issue.

When coverage binary is being used, this test case could fail with:

#  + mkdir /output
# + tar -cf /output/installer-iso.tar -C /stacker .
# tar: ./.coverage: file changed as we read it

The /stacker/.coverage file was being updated by the stacker binary itself. This is just a race condition based on when the file gets written to and when it was being collected by tar.

The change is just to only have tar collect the .cpio files.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a34ebfa) 54.24% compared to head (045d32a) 5.72%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #557 +/- ## ========================================== - Coverage 54.24% 5.72% -48.53% ========================================== Files 64 64 Lines 7473 7473 ========================================== - Hits 4054 428 -3626 - Misses 2728 6982 +4254 + Partials 691 63 -628 ```

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