Implement `Sync` for `process::Command`

[jonhoo]

std::process::Command is currently Send, but not Sync. While Sync probably isn't super useful for Command itself, it does mean that any type that wraps a process::Command also isn't Sync even if it might be useful on that type.

Command currently has a manual impl Send following #47760 (issue #47751), though from the discussion on users.r-l.o, it sounds like a manual impl Sync would also be sound. @cuviper not sure if that still holds?

If it does, this seems like a worthwhile impl to add, simply for the auto-trait implications.

[jonhoo]

To give one example of where this crops up: tokio::process::Command currently isn't Sync because std::process::Command isn't Sync. This also means that any async fn that uses tokio::process::Command across an await point is not Sync. Again, the ramifications of that probably aren't too severe (how important is it that a Future is Sync?), but it does lead to some annoyance when trying to use that future with libraries that just stick Send + Sync bounds wherever to get rid of compiler errors 😅

[cuviper]

As with Send, I think it's just the argv that isn't Sync, but that should be fine to add.