Fix build-shell-from-argv: argv joined without separator

ober

a7b8861b67317b4d1e63dd8d1eb8a692f6362993

diff --git a/lib/std/os/aproc.ss b/lib/std/os/aproc.ss
index 6c6f950..8eee1f6 100644
--- a/lib/std/os/aproc.ss
+++ b/lib/std/os/aproc.ss
@@ -907,8 +907,8 @@
                             (cond
                               [(null? as) (reverse acc)]
                               [else (lp (cdr as)
-                                        (cons (if (null? acc) "" " ")
-                                              (cons (sh-quote (car as)) acc)))])))]
+                                        (cons (sh-quote (car as))
+                                              (if (null? acc) acc (cons " " acc))))])))]
            [withdir (if dir (string-append "cd " (sh-quote dir) " && exec " quoted)
                       (string-append "exec " quoted))]
            [envprefix