[build] Fix join statement

bob/devtools/build.py

@@ -256,7 +256,7 @@ def exists_on_channel(channel_url, basename):
         candidate = urls[other_build_numbers.index(self_build_number)]
         pkg_type = '.conda' if basename.endswith('.conda') else '.tar.bz2'
         if candidate.endswith(pkg_type):  #match
-            return "".join(channel_url, candidate)
+            return "".join((channel_url, candidate))
 
 
 def remove_pins(deps):