build: move solve opt out of duplicate map

This was more error prone, as opposed to the approach used prior to
616fb3e55c.

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/driver.go

@@ -19,6 +19,8 @@ import (
 )
 
 type resolvedNode struct {
+	SolveOpt *client.SolveOpt
+
 	resolver    *nodeResolver
 	driverIndex int
 	platforms   []specs.Platform