[gdb/cli] Use captured per_command_time in worker threads

[gdb/cli] Use captured per_command_time in worker threads

With test-case gdb.base/maint.exp, I ran into:
...
(gdb) file maint^M
Reading symbols from maint...^M
(gdb) mt set per-command on^M
(gdb) Time for "DWARF indexing worker": ...^M
Time for "DWARF indexing worker": ...^M
Time for "DWARF indexing worker": ...^M
Time for "DWARF indexing worker": ...^M
Time for "DWARF skeletonless type units": ...^M
Time for "DWARF add parent map": ...^M
Time for "DWARF finalize worker": ...^M
Time for "DWARF finalize worker": ...^M
Time for "DWARF finalize worker": ...^M
Time for "DWARF finalize worker": ...^M
Time for "DWARF finalize worker": ...^M
FAIL: $exp: warnings: per-command: mt set per-command on (timeout)
mt set per-command off^M
2025-05-31 09:33:44.711 - command started^M
(gdb) PASS: $exp: warnings: per-command: mt set per-command off
...

I didn't manage to reproduce this by rerunning the test-case, but it's fairly
easy to reproduce using a file with more debug info, for instance gdb:
...
$ gdb -q -batch -ex "file build/gdb/gdb" -ex "mt set per-command on"
...

Due to the default "mt dwarf synchronous" == off, the file command starts
building the cooked index in the background, and returns immediately without
waiting for the result.

The subsequent "mt set per-command on" implies "mt set per-command time on",
which switches on displaying of per-command execution time.

The "Time for" lines are the result of those two commands, but these lines
shouldn't be there because "mt per-command time" == off at the point of
issuing the file command.

Fix this by capturing the per_command_time variable, and using the captured
value instead.

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
PR cli/33039
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33039