fix: temporary nexus binding should not overwrite permanent bind when given non-nexus position

[pythonmcpi]

See pull request title.
It might be a good idea to also wipe any existing temporary bind, but I don't see too much utility in implementing that and doing that would require more testing.

[Robotgiggle]

Removing an existing temp bind seems like it would be pretty easy. You could just use the same setup as lines 56-58 but remove the tag instead of setting it.

Also, regardless of whether you decide to do that or not, the new behavior should be documented in the book.

[pythonmcpi]

I'll add the temporary binding wiping. However, I would argue that this change is not new behavior, and simply changes existing behavior to align with player expectations (i.e. that the temporary binding pattern will never affect my permanently bound nexus).

[Robotgiggle]

Fair enough.