Support build where ~/ and $(pwd) are different filesystems

download_file caches original zips in ~/.gerritcodereview/buck-cache.
If this is a different filesystem than $(pwd)/buck-out a symlink
was used to connect the build file with the original archive. This
confuses Buck when creating the $SRCDIR for a genrule().

If a hardlink does not work, copy the file.

Change-Id: If66c59fb6aecc93b5a3e85cdd76ef880538875ff

tools/download_file.py

@@ -17,7 +17,7 @@ from __future__ import print_function
 
 from hashlib import sha1
 from optparse import OptionParser
-from os import link, makedirs, path, remove, symlink
+from os import link, makedirs, path, remove
 import shutil
 from subprocess import check_call, CalledProcessError
 from sys import stderr
@@ -178,4 +178,4 @@ else:
   try:
     link(cache_ent, args.o)
   except OSError as err:
-    symlink(cache_ent, args.o)
+    shutil.copyfile(cache_ent, args.o)