pgsql: ci: Improve OpenBSD core dump backtrace handling.

ci: Improve OpenBSD core dump backtrace handling.

Since OpenBSD core dumps do not embed executable paths, the script now
searches for the corresponding binary manually within the specified
directory before invoking LLDB.  This is imperfect but should find the
right executable in practice, as needed for meaningful backtraces.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAN55FZ36R74TZ8RKsFueYwLxGKDAm3LU2FHM_ZUCSB6imd3vYA@mail.gmail.com
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5114d62e74991151ea38a1a31e0806a9a78682d1

Modified Files
--------------
.cirrus.tasks.yml               |  3 ++-
src/tools/ci/cores_backtrace.sh | 31 +++++++++++++++++++++++++++----
2 files changed, 29 insertions(+), 5 deletions(-)