`env/prepend-to-pathlist` doesn't undo when it adds a duplicate

```
$ export VAR=a:b:c:d
$ cat inner/.shadowenv.d/500_custom.lisp
(env/prepend-to-pathlist "VAR" "c")
$ cd inner
activated shadowenv
$ echo $VAR
c:a:b:c:d
$ cd ..
$ echo $VAR
c:a:b:c:d
```

Undoing this properly would also require similar richer undo data including indexes as described in #16.