fix: support noxfile being a symlink

[henryiii]

Fix #828.

(Draft waiting for a test)

[jamesharris-garmin]

I actually use the existing behavior (see https://github.com/wntrblm/nox/issues/828#issuecomment-2147640069) but I can probably work around a change if this is more natural. I'll follow this so I know if this comes in and breaks me.

[henryiii]

Going to get it in for the next version; as seen in the tests, you can work around the change if you need the old behavior using __file__.