assemble-distribution: convert string relative bases to paths

Fixes an issue where the distribution assembler expects these to be
`path?`s.

I originally considered making this change in the ctool command in
`cext-lib` instead, but then I noticed the documented contract for
`assemble-distribution` is `(or/c #f path-string?)`.

racket/collects/compiler/distribute.rkt

@@ -138,7 +138,8 @@
         (let ([relative->binary-relative
                (lambda (sub-dir type relative-dir)
                  (cond
-                  [relative-base relative-base]
+                  [relative-base
+                   (if (string? relative-base) (string->path relative-base) relative-base)]
                   [(not executables?)
                    (build-path dest-dir relative-dir)]
                   [sub-dir