Re-enable ThinLTO on CI

[Mark-Simulacrum]

We previously disabled it on the dist bots, but this is currently untenable due to the length of time they run for. Should be relatively straightforward, we need to remove this line.

cc @kennytm @alexcrichton

[kennytm]

Do we need to reopen #45444 when this issue (#48222) is closed?

[Mark-Simulacrum]

I think we'll probably for now just take that perf hit on stable/beta releases but there's been some thought about disabling ThinLTO only for nightly, not stable/beta builds.

Edit: Rather, enabling ThinLTO only on nightly, and codegen-units=1 for stable/beta.