Fix soft link that not refer current file.

a6324cb forget fix soft link in ENV folder, it cause superenv use Apple provided git and svn.

For example, when checking out the svn repository, it will raise
```
svn: The path '/Library/Caches/Homebrew/<formula>--svn' appears to be part of a Subversion 1.7 or greater
working copy.  Please upgrade your Subversion client to use this
working copy.
```