refactor to fix empty image export/publishes

This is sorely lacking test coverage. Probably need a test that exports
and imports again.

But it's not possible to import a host path yet.

bass/bass.bass

@@ -59,7 +59,7 @@
         ($ cp -a & $submodule-cp-args)
         ($ go mod download)))))
 
-(provide [build smoke-test tests docs coverage]
+(provide [build dist smoke-test tests docs coverage]
   (use (*dir*/buildkit.bass))
 
   (defn dist [src version os arch]

cmd/bass/frontend.go

@@ -204,9 +204,6 @@ func frontendBuild(ctx context.Context, gw gwclient.Client) (*gwclient.Result, e
 	ib, err := builder.Build(
 		ctx,
 		thunk,
-		func(st llb.ExecState, sourcePath string) llb.State {
-			return st.Root()
-		},
 		false, // don't run any entrypoint
 	)
 	if err != nil {