coldcard: wallet info dlg: fix missing "Export for Coldcard" btn
regression from 711a325085
```
47.97 | E | plugin | Plugin error. plugin: coldcard, hook: wallet_info_buttons
Traceback (most recent call last):
File "...\electrum\electrum\plugin.py", line 261, in run_hook
r = f(*args)
File "...\electrum\electrum\plugins\hw_wallet\plugin.py", line 344, in wrapper
return func(self, *args, **kwargs)
File "...\electrum\electrum\plugins\coldcard\qt.py", line 51, in wallet_info_buttons
wallet = main_window.wallet
AttributeError: 'WalletInfoDialog' object has no attribute 'wallet'
```
7cf22d4b9f