Investigate Ubuntu Mate Software Boutique for installing Brave

[mbacchi]

We've seen a report that a user could not update Brave after installing from Ubuntu Mate Software Boutique, due to an expired GPG key issue:

 W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: brave-browser-apt-release.s3.brave.com bionic InRelease: The following signatures were invalid: EXPKEYSIG A8580BDC82D3DC6C Brave Software <support@brave.com>
W: Failed to fetch brave-browser-apt-release.s3.brave.com/dists/bionic/I…  The following signatures were invalid: EXPKEYSIG A8580BDC82D3DC6C Brave Software <support@brave.com>

This should be investigated to determine if this software installation method uses apt under the covers and whether it imports the key and updates the brave-keyring package as our signing keys get rotated.

We can download an ISO of Ubuntu Mate here to test this scenario with: https://ubuntu-mate.org/