Use Process.exec in shards run

[straight-shoota]

Resolves #685

[beta-ziliani]

Just noting that tests are failing

[straight-shoota]

We're stopping execution of the current program with exec directly after emitting the last log message. Because of that, the async log dispatcher doesn't run anymore and the message never gets written.

For now I'm adding a patch to explicitly close the log backend, which flushes all pending message.

But this seems like a general problem that should be addressed in the Crystal repo.

[straight-shoota]

The failure on Windows is due to crystal-lang/crystal#14422
So apparently we cannot use Process.exec on Windows just now.