[bevy-0.12] Migrate to Interned ScheduleLabels

[atlv24]

For Bevy 0.12

the only sketchy thing here is setup.rs:128

let fixed_update = schedule == FixedUpdate.intern();

[Jondolf]

Thanks! CI is failing, but it's probably because I haven't done the rest of the migration properly yet, and some things seem to have changed on Bevy main. I'll do it today though (Edit: I did in fact not do it that day, I was busy with other work. I'll do it very soon™ though)

[Jondolf]

I'll merge this since we need to do this for the migration anyway, and fix the remaining issues (+ verify the fixed update check) in the base migration PR

[Jondolf]

Btw I tested and the fixed update check does seem to work, it's consistently returning true when running physics in FixedUpdate and false otherwise