fix: race condition in ParseWithFuncs (#223)

Signed-off-by: Carlos A Becker <caarlos0@users.noreply.github.com>

env.go

@@ -72,8 +72,10 @@ var (
 			return float32(f), err
 		},
 	}
+)
 
-	defaultTypeParsers = map[reflect.Type]ParserFunc{
+func defaultTypeParsers() map[reflect.Type]ParserFunc {
+	return map[reflect.Type]ParserFunc{
 		reflect.TypeOf(url.URL{}): func(v string) (interface{}, error) {
 			u, err := url.Parse(v)
 			if err != nil {
@@ -89,7 +91,7 @@ var (
 			return s, err
 		},
 	}
-)
+}
 
 // ParserFunc defines the signature of a function that can be used within `CustomParsers`.
 type ParserFunc func(v string) (interface{}, error)
@@ -187,7 +189,7 @@ func ParseWithFuncs(v interface{}, funcMap map[reflect.Type]ParserFunc, opts ...
 	if ref.Kind() != reflect.Struct {
 		return ErrNotAStructPtr
 	}
-	parsers := defaultTypeParsers
+	parsers := defaultTypeParsers()
 	for k, v := range funcMap {
 		parsers[k] = v
 	}