Replace deprecated Ivar.fill with Ivar.fill_exn

[erenon]

On master, invoking make default results in:

File "src/trace.ml", line 481, characters 4-13:
481 |     Ivar.fill exited_ivar ();
          ^^^^^^^^^
Error (alert deprecated): Async.Ivar.fill
[since 2023-04] Use [fill_exn]

[Xyene]

The build failed, most likely because your environment has dependencies that are newer than this repository expects.

Did you create a new switch for magic-trace? See https://github.com/janestreet/magic-trace/wiki/Build-instructions for details.

[erenon]

Thanks for the quick reply! For some reason, I couldn't find the build instruction wiki page... Followed the steps in a new shell, and it compiles now.