Do not support pointer to user type

// use github.com/caarlos0/env/v11 v11.3.1
```
func TestCustomParserWIthDefault(t *testing.T) {
type foo struct {
name string
}

type bar struct {
Name string `env:"OTHER_CUSTOM"`
Foo *foo `env:"BLAH_CUSTOM"`
}

type config struct {
Var foo `env:"VAR_CUSTOM"`
Foo *foo `env:"BLAH_CUSTOM"`
Other *bar
}

t.Setenv("VAR_CUSTOM", "test")
t.Setenv("OTHER_CUSTOM", "test2")
t.Setenv("BLAH_CUSTOM", "test3")

runtest := func(t *testing.T) {
t.Helper()
cfg := &config{
Var: foo{"var"},
Foo: &foo{"foo"},
Other: &bar{
Name: "name",
Foo: &foo{"foo"},
},
}
err := ParseWithOptions(cfg, Options{FuncMap: map[reflect.Type]ParserFunc{
reflect.TypeOf(foo{}): func(v string) (interface{}, error) {
return foo{name: v}, nil
},
}})

isNoErr(t, err)
expect := &config{
Var: foo{"test"},
Foo: &foo{"test3"},
Other: &bar{
Name: "test2",
Foo: &foo{"test3"},
},
}
assert.Equal(t, expect, cfg)
}

for i := 0; i < 10; i++ {
t.Run(fmt.Sprintf("%d", i), runtest)
}
}
```
this test is failed
```
Diff:
--- Expected
+++ Actual
@@ -5,3 +5,3 @@
Foo: (*env.foo)({
- name: (string) (len=5) "test3"
+ name: (string) (len=3) "foo"
}),
@@ -10,3 +10,3 @@
Foo: (*env.foo)({
- name: (string) (len=5) "test3"
+ name: (string) (len=3) "foo"
})
```

this patch https://github.com/serg2014/env/commit/931c721de5f2c30e04742b1221fb44af9d736487
resolve problem

Start by running the supplied TestCustomParserWIthDefault reproducer and inspect ParseWithOptions with its FuncMap handling for pointer fields. Compare the referenced commit, then verify that custom parsing updates both direct and nested *foo fields to the environment values without changing non-pointer behavior.

Written by the indexing model from the issue text.