What about this patch?

Is there a way to compose the following part somehow to prevent a
possible race condition?

    (rename-file prog prog-real)

    (with-output-to-file prog (lambda ()
                                (format #t
                                        "#!~a~%~a~%exec ~a~%"
                                        (which "bash")
                                        (string-join (map export-variable vars)
                                                     "\n")
                                        (canonicalize-path prog-real))))
    (chmod prog #o755)))