unix: fix `gracket`-based launcher creation
unix: fix `gracket`-based launcher creation

The launcher was always create assuming a specific relative path
from the "bin" drectory to the "lib" directory, but that relative
path changes in a Unix-style install.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket