I tried building from source but got this error:
$ raco req -d file-exists?: contract violation expected: path-string? given: #f context...: /home/dave/.local/share/racket/8.8-guix/pkgs/req-lib/req/cmdline.rkt:87:25 .../racket/cmdline.rkt:191:51 body of "/home/dave/.local/share/racket/8.8-guix/pkgs/req-lib/req/raco-req.rkt" /gnu/store/2mhw66fpdlf5i6iamyc1zc4kvnj7k9lf-racket-vm-cs-8.8/opt/racket-vm/collects/raco/raco.rkt:41:0 body of "/gnu/store/2mhw66fpdlf5i6iamyc1zc4kvnj7k9lf-racket-vm-cs-8.8/opt/racket-vm/collects/raco/raco.rkt" body of "/gnu/store/2mhw66fpdlf5i6iamyc1zc4kvnj7k9lf-racket-vm-cs-8.8/opt/racket-vm/collects/raco/main.rkt"