GH-91048: Don't attempt to run on FreeBSD

[ambv]

The ifdef game was inadequate for our diverse set of buildbots.

• Issue: Async Call-Stack Reconstruction #91048

[ambv]

!buildbot FreeBSD

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @ambv for commit 25d2819 🤖

The command will test the builders whose names match following regular expression: FreeBSD

The builders matched are:

• AMD64 FreeBSD PR
• AMD64 FreeBSD Refleaks PR
• AMD64 FreeBSD14 PR
• AMD64 FreeBSD15 PR

[vstinner]

test_external_inspection fails on AMD64 Fedora Stable LTO + PGO 3.x: https://buildbot.python.org/#/builders/29/builds/7598

And on AMD64 RHEL8 LTO 3.x: https://buildbot.python.org/#/builders/64/builds/8461

Example:

ERROR: test_async_gather_remote_stack_trace (test.test_external_inspection.TestGetStackTrace.test_async_gather_remote_stack_trace)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.x.cstratak-fedora-stable-x86_64.lto-pgo/build/Lib/test/test_external_inspection.py", line 271, in test_async_gather_remote_stack_trace
    stack_trace = get_async_stack_trace(p.pid)
SystemError: <built-in function get_async_stack_trace> returned NULL without setting an exception

read_async_debug() returns NULL without setting an exception: get_async_debug(pid) returns -1 because search_map_for_section() returns NULL simply because well, it didn't find the section.

cc @encukou