only validate root snapshot when loading file (#6057)

Currently, we validate the root snapshot every time we startup, meaning
that for all startups after the first, we are validating the same thing
that we wrote to disk.

This causes a problem on v0.33, because some parts of the software
expect a sealing segment of length 590+, and others expect a sealing
segment of length 600+. In particular, this can cause a node to accept a
root snapshot file when bootstrapping, then reject the root snapshot
DB-backed object on subsequent startups, because the DB-backed snapshot
is returning an invalid sealing segment.

cmd/scaffold.go

@@ -1330,6 +1330,13 @@ func (fnb *FlowNodeBuilder) initState() error {
 				return fmt.Errorf("failed to read protocol snapshot from disk: %w", err)
 			}
 		}
+		// apply any extra validation steps to the root snapshot prior to bootstrapping
+		if fnb.extraRootSnapshotCheck != nil {
+			err = fnb.extraRootSnapshotCheck(rootSnapshot)
+			if err != nil {
+				return fmt.Errorf("failed validate root snapshot from disk: %w", err)
+			}
+		}
 		// set root snapshot fields
 		if err := fnb.setRootSnapshot(rootSnapshot); err != nil {
 			return err
@@ -1412,14 +1419,6 @@ func (fnb *FlowNodeBuilder) setRootSnapshot(rootSnapshot protocol.Snapshot) erro
 		return fmt.Errorf("failed to validate root snapshot QCs: %w", err)
 	}
 
-	// perform extra checks requested by specific node types
-	if fnb.extraRootSnapshotCheck != nil {
-		err = fnb.extraRootSnapshotCheck(rootSnapshot)
-		if err != nil {
-			return fmt.Errorf("failed to perform extra checks on root snapshot: %w", err)
-		}
-	}
-
 	fnb.RootSnapshot = rootSnapshot
 	// cache properties of the root snapshot, for convenience
 	fnb.RootResult, fnb.RootSeal, err = fnb.RootSnapshot.SealedResult()