fix lease management with flightcontrol

[tonistiigi]

fixes #4419

When one flightcontrol callback gets canceled
ctx.Value() stops working for aquiring leases for
remaining callbacks. While this behavior should be also looked at more carefully, returning a lease for the first callback or for the remaining callback would not be correct as some objects can be tracked by first lease and that lease could be already deleted by the first callpath.

This fixes it so that any object tracked by flightcontrol callback will be copied to the lease of every codepath after the callback has returned.

[tonistiigi]

@ktock Can you take a quick look at these "missing lease requirement" errors for stargz. I can easily remove this validation in Adopt() if leases are not needed for these functions but I don't want to hide any other possible bug if these code paths should actually have leases set in context. Atm. it seems to depend on the code path if a lease is set or not.

[ktock]

Thanks for pinging me. According to the stack trace, the error comes from github.com/moby/buildkit/cache.(*immutableRef).prepareRemoteSnapshotsStargzMode and this function only creates snapshots that are traced by the ref's lease so I think it's OK the input ctx doesn't have lease.