MDBF-1194: Reduce the memory pressure of hz-bbw8 and hz-bbw9

During the MTR phase of amd64-msan-clang-20-debug, the "MTR normal"
step intermittently reports success even when errors are present
in the stdio output.

Observed symptoms:
- truncated MTR logs in stdio
- step completes with exit code 0 despite failures

System logs show evidence of OOM killer activity during these runs.
In particular, addr2line processes spawned while generating backtraces
are being killed:

```
Mar 18 02:55:49 hz-bbw8 kernel: checkpoint_bg invoked oom-killer: gfp_mask=0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), order=0, oom_score_adj=0
Mar 18 02:55:49 hz-bbw8 kernel:  oom_kill_process.cold+0xb/0x10
Mar 18 02:55:49 hz-bbw8 kernel: [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
Mar 18 02:55:50 hz-bbw8 kernel: oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=docker-15b53e20d1e4f7dba6166ebce83a54eaa35e13a2460cd0647ef632639ad6a95b.scope,mems_allowed=0,global_oom,task_memcg=/system.slice/docker-15b53e20d1e4f7dba6166ebce83a54eaa35e13a2460cd0647ef632639ad6a95b.scope,task=addr2line,pid=212899,uid=1000
Mar 18 02:55:50 hz-bbw8 kernel: Out of memory: Killed process 212899 (addr2line) total-vm:14357312kB, anon-rss:14320664kB, file-rss:3840kB, shmem-rss:0kB, UID:1000 pgtables:28136kB oom_score_adj:0
Mar 18 02:55:52 hz-bbw8 kernel: oom_reaper: reaped process 212899 (addr2line), now anon-rss:0kB, file-rss:3840kB, shmem-rss:0kB
```

The fact that most of the runs end while attempting a backtrace is not a coincidence.

```
Retrying test innodb_fts.innodb_fts_large_records, attempt(2/3)...
***Warnings generated in error logs during shutdown after running tests: innodb_fts.innodb_fts_large_records
260318 10:24:04 [ERROR] /home/buildbot/bld/sql/mariadbd got signal 6 ;
Attempting backtrace. Include this in the bug report.
```

This suggests that MTR does not properly propagate failures when
child processes are terminated by the OOM killer, resulting in the
main perl process exiting with code 0.

This change documents the issue and is an attempt at avoiding the issue
by reducing the number of parallel workers at runtime.

Author: RazvanLiviuVarzaru

master-migration/master.cfg

@@ -193,7 +193,7 @@ def msan_builder(name: str, debug: bool) -> GenericBuilder:
     jobs = 12
     if debug:
         tag_msan = tags_msan + ("debug",)
-        jobs = 20
+        jobs = 15
 
     return GenericBuilder(
         name=name,