Fix find_placement function using addon_place=True

[BurnySc2]

Fixes #223
Related to 1ec2d72

This change fixes find_placement function to work properly with addon_place=True

It can be tested by running the terran/onebase_battlecruiser.py example with the following code change:

if self.can_afford(UnitTypeId.STARPORT):
    position = await self.find_placement(
        UnitTypeId.STARPORT,
        near=cc.position - (3, 0),
        placement_step=1,
        addon_place=True,
    )
    if position is not None:
        await self.build(
            UnitTypeId.STARPORT,
            near=position,
        )

The - (3, 0) forces the function to find a valid position on the left of the main command center.