unbreak benchmark starters

pkgs/racket-benchmarks/tests/racket/benchmarks/common/auto.rkt

@@ -432,7 +432,7 @@ exec racket -qu "$0" ${1+"$@"}
                 void
                 mk-racket
                 (lambda (bm)
-                  (system* (or (hash-ref executables "racket") (find-exe)) "-u" (compiled-path bm)))
+                  (system* (or (hash-ref executables "racket" #f) (find-exe)) "-u" (compiled-path bm)))
                 extract-racket-times
                 clean-up-zo
                 racket-skip-progs)

pkgs/racket-benchmarks/tests/racket/benchmarks/rx/auto.rkt

@@ -287,7 +287,9 @@ exec racket -qu "$0" ${1+"$@"}
 
   (define-values (actual-benchmarks-to-run 
                   actual-implementations-to-run 
-                  num-iterations)
+                  num-iterations
+                  executables
+                  names)
     (process-command-line benchmark-names null
                           (map car testers) non-defaults
                           3))