bake: fix map type checking to detect matrix property set to list

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

bake/bake.go

@@ -820,7 +820,7 @@ func (t *Target) GetEvalContexts(ectx *hcl.EvalContext, block *hcl.Block, loadDe
 		return nil, err
 	}
 
-	if !value.CanIterateElements() {
+	if !value.Type().IsMapType() && !value.Type().IsObjectType() {
 		return nil, errors.Errorf("matrix must be a map")
 	}
 	matrix := value.AsValueMap()