我意外删除了 ubuntu 中的 /var 文件夹，现在我正尝试备份并重新安装 ubuntu，按照本网站的步骤操作：https://linuxconfig.org/ubuntu-20-04-system-backup-and-restore。但是，当我尝试安装 timeshift ( sudo apt install timeshift) 时，我得到了E: Unable to locate package timeshift。当我运行时，sudo apt update我得到了这：
Get:1 http://repo.mysql.com/apt/ubuntu focal InRelease [12.9 kB]
Err:1 http://repo.mysql.com/apt/ubuntu focal InRelease
Couldn't create temporary file /tmp/apt.conf.7WSRQF for passing config to apt-key
Get:2 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
Get:3 http://ppa.launchpad.net/linrunner/tlp/ubuntu focal InRelease [17.5 kB]
Err:3 http://ppa.launchpad.net/linrunner/tlp/ubuntu focal InRelease
Couldn't create temporary file /tmp/apt.conf.oYNeaO for passing config to apt-key
Get:4 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Get:5 http://ppa.launchpad.net/x2go/stable/ubuntu focal InRelease [23.8 kB]
Err:5 http://ppa.launchpad.net/x2go/stable/ubuntu focal InRelease
Couldn't create temporary file /tmp/apt.conf.uvGnpS for passing config to apt-key
Err:2 http://security.ubuntu.com/ubuntu focal-security InRelease
Couldn't create temporary file /tmp/apt.conf.de91MW for passing config to apt-key
Err:4 http://archive.ubuntu.com/ubuntu focal InRelease
Couldn't create temporary file /tmp/apt.conf.f6AojY for passing config to apt-key
Get:6 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Err:6 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Couldn't create temporary file /tmp/apt.conf.NR1QN2 for passing config to apt-key
Get:7 http://archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB]
Err:7 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Couldn't create temporary file /tmp/apt.conf.XvVS49 for passing config to apt-key
Reading package lists... Done
W: GPG error: http://repo.mysql.com/apt/ubuntu focal InRelease: Couldn't create temporary file /tmp/apt.conf.7WSRQF for passing config to apt-key
E: The repository 'http://repo.mysql.com/apt/ubuntu focal 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.
W: GPG error: http://ppa.launchpad.net/linrunner/tlp/ubuntu focal InRelease: Couldn't create temporary file /tmp/apt.conf.oYNeaO for passing config to apt-key
E: The repository 'http://ppa.launchpad.net/linrunner/tlp/ubuntu focal 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.
W: GPG error: http://ppa.launchpad.net/x2go/stable/ubuntu focal InRelease: Couldn't create temporary file /tmp/apt.conf.uvGnpS for passing config to apt-key
E: The repository 'http://ppa.launchpad.net/x2go/stable/ubuntu focal 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.
W: GPG error: http://security.ubuntu.com/ubuntu focal-security InRelease: Couldn't create temporary file /tmp/apt.conf.de91MW for passing config to apt-key
E: The repository 'http://security.ubuntu.com/ubuntu focal-security 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.
W: GPG error: http://archive.ubuntu.com/ubuntu focal InRelease: Couldn't create temporary file /tmp/apt.conf.f6AojY for passing config to apt-key
E: The repository 'http://archive.ubuntu.com/ubuntu focal 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.
W: GPG error: http://archive.ubuntu.com/ubuntu focal-updates InRelease: Couldn't create temporary file /tmp/apt.conf.NR1QN2 for passing config to apt-key
E: The repository 'http://archive.ubuntu.com/ubuntu focal-updates 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.
W: GPG error: http://archive.ubuntu.com/ubuntu focal-backports InRelease: Couldn't create temporary file /tmp/apt.conf.XvVS49 for passing config to apt-key
E: The repository 'http://archive.ubuntu.com/ubuntu focal-backports 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.
我检查了该软件包是否适用于我的 ubuntu 版本这里，是的，确实如此。universe 存储库也已启用。我尝试安装 shutter 和 nmap，但出现了同样的错误。我真的很感激有人能帮我。谢谢。