Piped function alias with THREAD_SUBPROCS=False gets stuck

[daniel-shimon]

The following gets stuck forever:

$THREAD_SUBPROCS=False
aliases['a'] = lambda: ![echo hi]
a | grep h

This seems to happen in 0.9.27 as well (meaning $XONSH_CAPTURE_ALWAYS isn't the cause).
The problem probably comes from somewhere in ProcProxy.

[Mic92]

This also happens if bashims is load:

xonsh/xontrib/bashisms.py

Line 86 in 15ff663

XSH.env["THREAD_SUBPROCS"] = False