我想安装该包斯勒在 Debian 上。它在 Ubuntu 上作为 PPA 提供。我找到了该网址并将其添加到我的sources.list文件中。然后我更新了。但sile由于错误，无法下载。
$ printf "deb https://ppa.launchpadcontent.net/sile-typesetter/sile/ubuntu kinetic main\ndeb-src https://ppa.launchpadcontent.net/sile-typesetter/sile/ubuntu kinetic main\n" >> /etc/apt/sources.list
$ sudo apt update
Get:15 https://ppa.launchpadcontent.net/sile-typesetter/sile/ubuntu kinetic InRelease [18.1 kB]
Err:15 https://ppa.launchpadcontent.net/sile-typesetter/sile/ubuntu kinetic InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 904F67626F1CF535
Reading package lists... Done
W: GPG error: https://ppa.launchpadcontent.net/sile-typesetter/sile/ubuntu kinetic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 904F67626F1CF535
E: The repository 'https://ppa.launchpadcontent.net/sile-typesetter/sile/ubuntu kinetic 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.
这样做真的是“不安全”且不好的做法吗？如果我还想继续，我该怎么办？