move CloneWith() call into interpolate()

interpolate.go

@@ -8,9 +8,18 @@ import (
 var interpolationRegex = regexp.MustCompile(`((?:\${([[:alpha:]_][[:word:]]*))(?:=([^}]+))?})|(\$)|([^$]+)`)
 
 // Interpolate variables from vars into s for substrings in the form ${var} or ${var=default}.
-func interpolate(s string, vars map[string]string) (string, error) {
+func interpolate(s string, vars Vars, updatedVars map[string]string) (string, error) {
 	out := ""
 	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
+	if len(matches) == 0 {
+		return s, nil
+	}
+	for key, val := range updatedVars {
+		if vars[key] != val {
+			vars = vars.CloneWith(updatedVars)
+			break
+		}
+	}
 	for _, match := range matches {
 		if name := match[2]; name != "" {
 			value, ok := vars[name]