$REPOS
refers to the location of the whole repository and not a checked out copy. The Subversion server cannot possibly know where the client is going to write the data.
It is also not possible to modify the repository itself upon committing (as modifying a file).
If you are the only user of the repository and you always check out the stuff in the same sandbox you can hard-code the location in your post-commit hook
/usr/bin/svnlook info $REPOS >> /the/location/of/my/sandbox/changelog.txt