reproducible: apache: use the correct variable to match possible existing files on disk

REQUEST_FILENAME may or may not be a full pathname, depending on when
it's expanded.

From the docs:
    REQUEST_FILENAME
    The full local filesystem path to the file or script matching the
    request, if this has already been determined by the server at the
    time REQUEST_FILENAME is referenced. Otherwise, such as when used in
    virtual host context, the same value as REQUEST_URI

Since we are using virtual hosts (like, uh, everybody since apache 2.2?!)
REQUEST_FILENAME is not much of use.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>