print_exception and threaded alias: no traceback on second run

[anki-code]

I found weird artifact related to this PR during work on #5366.
Minimal repeatable example:

# docker run --rm -it xonsh/xonsh:slim xonsh

xonfig
# xonsh 0.14.0, python 3.11

$RAISE_SUBPROC_ERROR = False
$XONSH_SHOW_TRACEBACK = False
from xonsh.tools import print_exception
def _f():
    try:
       1/0
    except:
       print_exception(f"Exception")
aliases['f'] = _f

f
# ZeroDivisionError: division by zero
# Exception

f
# <----------------- Empty!

f
# ZeroDivisionError: division by zero
# Exception

After diving into why the second stderr output is forever empty I see that if I remove this line from print_exception() everything is good:

xonsh/xonsh/tools.py

Lines 1056 to 1058 in 55b341d

	sys.last_type, sys.last_value, sys.last_traceback = exc_info

If this line is working I see that only on the second run of alias the stderr descriptor raises silent error on flushing:

xonsh/xonsh/procs/proxies.py

Line 565 in 55b341d

safe_flush(sp_stderr)

xonsh/xonsh/procs/proxies.py

Lines 40 to 47 in 55b341d

	def safe_flush(handle):
	"""Attempts to safely flush a file handle, returns success bool."""
	status = True
	try:
	handle.flush()
	except OSError:
	status = False
	return status

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