fix: interpolate longName and shortName in PKI backup download

This caused {{shortName}} and {{longName}} to appear unformatted in the
exported key files:

``` === MESHTASTIC KEYS FOR {{longName}} ({{shortName}}) ===

Private Key: <censored>

Public Key: <censored>

=== END OF KEYS === ```

The fix simply replicates the behaviour used elsewhere in
PKIIBackupDialog.