Fix command to install box86 keyring.

[geerlingguy]

Without this fix, you wind up getting:

W: GPG error: https://itai-nelken.github.io/weekly-box86-debs/debian  InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EB3CE9A337ECDFA4
E: The repository 'https://itai-nelken.github.io/weekly-box86-debs/debian  InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

See the official box86 setup instructions: https://itai-nelken.github.io/weekly-box86-debs/