Prepend jinja2 path for hermeticity.

On systems with an older local install, appending the third_party path to
jinja2 will result in the local install being used.
Switching to prepending to avoid this.

BUG=470543
TEST=local
R=ncbray@chromium.org,jamesr@chromium.org

Review URL: https://codereview.chromium.org/813483005

Cr-Commit-Position: refs/heads/master@{#322712}

mojo/nacl/generator/generate_nacl_bindings.py

@@ -29,7 +29,7 @@ def _GetDirAbove(dirname):
 
 
 def _AddThirdPartyImportPath():
-  sys.path.append(os.path.join(_GetDirAbove('mojo'), 'third_party'))
+  sys.path.insert(0, os.path.join(_GetDirAbove('mojo'), 'third_party'))
 
 
 _AddThirdPartyImportPath()