fix: write appdmg.json to a unique tmpdir path

[malept]

Closes #34

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@42793b7). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #74   +/-   ##
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  Coverage        ?   75.43%           
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  Files           ?        1           
  Lines           ?       57           
  Branches        ?        0           
=======================================
  Hits            ?       43           
  Misses          ?       14           
  Partials        ?        0           

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