Fix file permissions when extracting zip files (#325)

This means that `./emsdk` works on UNIX system after emsdk self-updates
from a zip file.  Without this one would need to run `python ./emsdk`
which seems to be why the tests were doing it this way.

test.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python
 import json
 import os
 import shlex
@@ -149,13 +150,13 @@ temp_dir = tempfile.mkdtemp()
 
 for filename in os.listdir('.'):
   if not filename.startswith('.') and not os.path.isdir(filename):
-    shutil.copyfile(filename, os.path.join(temp_dir, filename))
+    shutil.copy2(filename, os.path.join(temp_dir, filename))
 
 os.chdir(temp_dir)
 
-check_call('python ./emsdk update')
+check_call('./emsdk update')
 print('second time')
-check_call('python ./emsdk update')
+check_call('./emsdk update')
 
 print('verify downloads exist for all OSes')
 latest_hash = TAGS['releases'][TAGS['latest']]