download: Don't abbreviate things that are not store items.

Fixes a regression introduced in a8be7b9a.

* guix/build/download.scm (store-path-abbreviation): Return STORE-PATH
  if it's not an actual store path.  Fixes an out-of-range exception
  when running tests/substitute.scm and tests/store.scm.

guix/build/download.scm

@@ -111,12 +111,15 @@ (define (string-pad-middle left right len)
     (string-append left padding right)))
 
 (define* (store-path-abbreviation store-path #:optional (prefix-length 6))
-  "Return an abbreviation of STORE-PATH for display, showing PREFIX-LENGTH
-characters of the hash."
-  (let ((base (basename store-path)))
-    (string-append (string-take base prefix-length)
-                   "…"
-                   (string-drop base 32))))
+  "If STORE-PATH is the file name of a store entry, return an abbreviation of
+STORE-PATH for display, showing PREFIX-LENGTH characters of the hash.
+Otherwise return STORE-PATH."
+  (if (string-prefix? (%store-directory) store-path)
+      (let ((base (basename store-path)))
+        (string-append (string-take base prefix-length)
+                       "…"
+                       (string-drop base 32)))
+      store-path))
 
 (define* (progress-proc file size
                         #:optional (log-port (current-output-port))