Use clone instead of fork on pvf (#2477)

@mrcnski Done the change on the prepare worker, once the prepare worker
part is good I'll do the same for the execute worker.

This is based on
https://github.com/koute/polkavm/blob/11beebd06276ce9b84f335350138479e714f6caf/crates/polkavm/src/sandbox/linux.rs#L711.

## TODO

- [x] Add a check for this capability at startup
- [x] Add prdoc mentioning the new Secure Validator Mode (optional)
requirement.

## Related

Closes #2162

---------

Co-authored-by: Marcin S <marcin@realemail.net>

polkadot/node/core/pvf/src/host.rs

@@ -906,7 +906,7 @@ pub(crate) mod tests {
 			let _ = pulse.next().await.unwrap();
 
 			let el = start.elapsed().as_millis();
-			assert!(el > 50 && el < 150, "{}", el);
+			assert!(el > 50 && el < 150, "pulse duration: {}", el);
 		}
 	}