Fix codesign for MacOS build

Apple goes like “the bundle name says it belongs to Unity. But the bundle signature doesn’t say that, so it’s invalid”

We have a problem with the codesign, we should be signing with our own identity and not '-'

Currently, the issue this causes is that developers are forced to open the UnityPurchasing.bundle, edit the CFBundleIdentifier to change it:
https://discussions.unity.com/t/cfbundleidentifier-collision-com-unity-purchasing-unitypurchasing/838590/58

If we sign this properly, developers won't have to do this step (we want to update our docs after this is fixed).

We can ask for additional help on #devs-code-signing if this isn't straightforward to fix.