build: move solveOpt to local struct type

*client.SolveOpt in driver code is only used by build code.
For a clear separation of concerns, move it to an internal
struct type only accessible by BuildWithResultHandler func.

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

build/driver.go

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