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.sh

@@ -1,3 +1,4 @@
+#!/usr/bin/env sh
 echo "test the standard workflow (as close as possible to how a user would do it, in the shell)"
 ./emsdk install latest
 ./emsdk activate latest