I copied your functions and did this in my own editor and it worked as anticipated, prints 1 then 0:
var save = 0
func _ready() -> void:
save(save)
save = 1
print(save)
load_()
print(save)
Try changing user://savegame.save
to res://savegame.save
in both functions and see if that gives you any results. If that works you have a permissions issue in the directory that user://
points to. If it doesn't work, i think you have a permissions issue overall.