apport: get value for CoreDump only once

[bdrung]

Ease mypy to determine the type of the problem report value for CoreDump by getting the value only once and store it in a variable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.56%. Comparing base (1e03e7c) to head (8593aa3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #612   +/-   ##
=======================================
  Coverage   84.56%   84.56%           
=======================================
  Files         104      104           
  Lines       21028    21029    +1     
  Branches     3219     3219           
=======================================
+ Hits        17782    17783    +1     
+ Misses       2805     2804    -1     
- Partials      441      442    +1     

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[bdrung]

Just rebased.