Quote PATH when writing agent invocation script

If the PATH contains spaces, the agent invocation script will fail parsing. This quotes the variable so that spaces don't break the script.

libagent/gpg/__init__.py

@@ -143,7 +143,7 @@ def run_init(device_type, args):
     # Prepare GPG agent invocation script (to pass the PATH from environment).
     with open(os.path.join(homedir, 'run-agent.sh'), 'w') as f:
         f.write(r"""#!/bin/sh
-export PATH={0}
+export PATH="{0}"
 {1} \
 -vv \
 --pin-entry-binary={pin_entry_binary} \