clean.do: cleanup *.tmp files that might have been left lying around.

...and fix a bug where builder.py can't handle it if its temp file is
deleted out from under it.

builder.py

@@ -211,7 +211,13 @@ class BuildJob:
                 os.rename(self.tmpname2, t)
                 os.unlink(self.tmpname1)
             elif st1.st_size > 0:
-                os.rename(self.tmpname1, t)
+                try:
+                    os.rename(self.tmpname1, t)
+                except OSError, e:
+                    if e.errno == errno.ENOENT:
+                        unlink(t)
+                    else:
+                        raise
                 if st2:
                     os.unlink(self.tmpname2)
             else: # no output generated at all; that's ok