Fixed SCRIPT_DIR computation when CDPATH is set.

When `CDPATH` is set, the `cd` command echoes the directory where it actually went. In this case, the `SCRIPT_DIR` variable will contain the path twice: `/path/to/release/dir /path/to/release/dir` instead of `/path/to/release/dir`.

This fix unset `CDPATH` when computing `SCRIPT_DIR`.