ci: Improve OpenBSD core dump backtrace handling

Hi,

While working on [1], I realized that the core dump output of OpenBSD
has very little information. We had an off-list chat with Thomas and
found that this happens because OpenBSD core dump files do not include
the executable information; so we need to run `lldb` with the
executable path. Attached patch manually finds executables in the
build directory and calls `lldb` with these executables' paths'.

Here is an example:

Master [2]:
```
[11:36:51.769] (lldb) target create --core "/var/crash/postgres.core"
[11:36:51.797] Core file '/var/crash/postgres.core' (x86_64) was loaded.
[11:36:51.797] (lldb) thread backtrace all
[11:36:51.798] * thread #1, stop reason = signal SIGABRT
[11:36:51.798] * frame #0: 0x0000029d071e14bb
[11:36:51.798] frame #1: 0x550327f2b69964ee
[11:36:51.798] (lldb) quit
```

Patched [3]:
```
[11:22:11.087] (lldb) target create
"/home/postgres/postgres/build/tmp_install/usr/local/pgsql/bin/postgres"
--core "/var/crash/postgres.core"
[11:22:13.104] Core file '/var/crash/postgres.core' (x86_64) was loaded.
[11:22:13.104] (lldb) thread backtrace all
[11:22:13.105] warning: This version of LLDB has no plugin for the
language "assembler". Inspection of frame variables will be limited.
[11:22:13.213] * thread #1, stop reason = signal SIGABRT
[11:22:13.213] * frame #0: 0x00000b12e4dfe27b
libc.so.100.3`_thread_sys_thrkill at -:2
[11:22:13.213] frame #1: 0x00000b12e4db010b
libc.so.100.3`_libc_abort at abort.c:51:8
[11:22:13.213] frame #2: 0x00000b126d3fd39d
pg_prewarm.so`pg_prewarm(fcinfo=<unavailable>) at pg_prewarm.c:80:2
[11:22:13.213] frame #3: 0x00000b0ff584add5
postgres`ExecInterpExpr(state=0x00000b12527e4790,
econtext=0x00000b12527e4438, isnull=0x0000000000000000) at
execExprInterp.c:926:8
[11:22:13.213] frame #4: 0x00000b0ff5896a47 postgres`ExecResult
[inlined] ExecEvalExprNoReturn(state=0x00000b12527e4790,
econtext=0x00000b12527e4438) at executor.h:420:13
[11:22:13.213] frame #5: 0x00000b0ff5896a3b postgres`ExecResult
[inlined] ExecEvalExprNoReturnSwitchContext(state=0x00000b12527e4790,
econtext=0x00000b12527e4438) at executor.h:461:2
[11:22:13.213] frame #6: 0x00000b0ff5896a2a postgres`ExecResult
[inlined] ExecProject(projInfo=0x00000b12527e4788) at executor.h:493:2
[11:22:13.213] frame #7: 0x00000b0ff5896a11
postgres`ExecResult(pstate=<unavailable>) at nodeResult.c:135:10
[11:22:13.213] frame #8: 0x00000b0ff5855eda
postgres`standard_ExecutorRun [inlined]
ExecProcNode(node=0x00000b12527e4328) at executor.h:316:9
[11:22:13.213] frame #9: 0x00000b0ff5855ec5
postgres`standard_ExecutorRun [inlined]
ExecutePlan(queryDesc=0x00000b124f332500, operation=CMD_SELECT,
sendTuples=<unavailable>, numberTuples=0, direction=<unavailable>,
dest=0x00000b12e5cae7a0) at execMain.c:1697:10
[11:22:13.213] frame #10: 0x00000b0ff5855e4a
postgres`standard_ExecutorRun(queryDesc=0x00000b124f332500,
direction=<unavailable>, count=0) at execMain.c:366:3
[11:22:13.213] frame #11: 0x00000b0ff5a7623b
postgres`PortalRunSelect(portal=0x00000b128d07c100,
forward=<unavailable>, count=0, dest=<unavailable>) at pquery.c:921:4
[11:22:13.213] frame #12: 0x00000b0ff5a75d9d
postgres`PortalRun(portal=0x00000b128d07c100,
count=9223372036854775807, isTopLevel=<unavailable>,
dest=0x00000b12e5cae7a0, altdest=0x00000b12e5cae7a0,
qc=0x0000759da39141b0) at pquery.c:765:18
[11:22:13.213] frame #13: 0x00000b0ff5a74ad9
postgres`exec_simple_query(query_string="SELECT pg_prewarm('test',
'read');") at postgres.c:1279:10
[11:22:13.213] frame #14: 0x00000b0ff5a72332
postgres`PostgresMain(dbname=<unavailable>, username=<unavailable>) at
postgres.c:0
[11:22:13.213] frame #15: 0x00000b0ff5a6bfad
postgres`BackendMain(startup_data=<unavailable>,
startup_data_len=<unavailable>) at backend_startup.c:124:2
[11:22:13.213] frame #16: 0x00000b0ff59a35a6
postgres`postmaster_child_launch(child_type=B_BACKEND, child_slot=1,
startup_data=0x0000759da3915030, startup_data_len=24,
client_sock=0x0000759da3914f20) at launch_backend.c:268:3
[11:22:13.213] frame #17: 0x00000b0ff59a8584 postgres`ServerLoop
[inlined] BackendStartup(client_sock=<unavailable>) at
postmaster.c:3598:8
[11:22:13.213] frame #18: 0x00000b0ff59a8543 postgres`ServerLoop
at postmaster.c:1713:6
[11:22:13.213] frame #19: 0x00000b0ff59a60e8
postgres`PostmasterMain(argc=<unavailable>, argv=<unavailable>) at
postmaster.c:1403:11
[11:22:13.213] frame #20: 0x00000b0ff58c6915 postgres`main(argc=4,
argv=0x0000759da39155c8) at main.c:231:4
[11:22:13.213] frame #21: 0x00000b0ff565728b postgres`__start + 299
```

Any feedback would be appreciated.

[1] https://postgr.es/m/CAN55FZ12R-LG=wZZ8yLY7+tmVbR=9Y_O5uveDHU0nmYT+AiTjQ@mail.gmail.com
[2] https://cirrus-ci.com/task/6177664672727040
[3] https://cirrus-ci.com/task/5487875443130368

--
Regards,
Nazir Bilal Yavuz
Microsoft