use Once instead of Mutex to manage capture resolution
For #78299
This allows us to return borrows of the captured backtrace frames that are tied to a borrow of the Backtrace itself, instead of to some short-lived Mutex guard.
We could alternatively share `&Mutex<Capture>`s and lock on-demand, but then we could potentially forget to call `resolve()` before working with the capture. It also makes it semantically clearer what synchronization is needed on the capture.
cc `@seanchen1991` `@rust-lang/project-error-handling`
e73ee1dde2