[9.18] Look for core files in $TOP_BUILDDIR

Merged Michal Nowak requested to merge mnowak/look-for-core-files-in-TOP_BUILDDIR-9.18 into bind-9.18

The get_core_dumps.sh script couldn't find and process core files of out-of-tree configurations because it looked for them in the source instead of the build directory.

(cherry picked from commit a13448a7)

Backport of MR !7869 (merged)

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