Repro for _concat not forwarding for_signature to generators

gistfile1.txt

import atexit

calls = 0
signature_calls = 0

def some_generator(target, source, env, for_signature):
    global calls, signature_calls
    calls += 1
    if for_signature:
        signature_calls += 1
    return "magic"


env = DefaultEnvironment()

env['GEN'] = some_generator
gen1 = env.Command(
    target="#genny1",
    source=[],
    action="echo $GEN",
)
env.Pseudo(gen1)
env.Alias('good', gen1)

env['GENLIST'] = ['$GEN']
env['GENPREFIX'] = 'PRE'
env['GENSUFFIX'] = 'SUF'
env['CONCAT_GEN'] = '$( ${_concat(GENPREFIX, GENLIST, GENSUFFIX, __env__, lambda x: x, TARGET, SOURCE)} $) '

gen2 = env.Command(
    target="#genny2",
    source=[],
    action="echo $CONCAT_GEN",
)
env.Pseudo(gen2)
env.Alias('bad', gen2)

env.Default(None)

def print_counts():
    global calls, signature_calls
    print('some_generator was called', calls)
    print('some_generaator was called for sig', signature_calls)

atexit.register(print_counts)