我按照此处和此处的说明安装了 TeXLive ，安装程序告诉我我正在安装 2020 版本：
Loading http://mirror.koddos.net/CTAN/systems/texlive/tlnet/tlpkg/texlive.tlpdb
Installing TeX Live 2020 from: http://mirror.koddos.net/CTAN/systems/texlive/tlnet (verified)
...
但是，询问tex --version返回给我：
TeX 3.14159265 (TeX Live 2017/Debian)
kpathsea version 6.2.3
Copyright 2017 D.E. Knuth.
There is NO warranty. Redistribution of this software is
covered by the terms of both the TeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the TeX source.
Primary author of TeX: D.E. Knuth.
我按照此处的建议更改了 tlmgr 的存储库：
sudo tlmgr option repository ftp://tug.org/historic/systems/texlive/2017/tlnet-final
但是现在给了我不再支持 2017 的错误。这就是我无法安装软件包的原因：
~$ tlmgr update --self
(running on Debian, switching to user mode!)
TeX Live 2017 is frozen forever and will no
longer be updated. This happens in preparation for a new release.
If you're interested in helping to pretest the new release (when
pretests are available), please read http://tug.org/texlive/pretest.html.
Otherwise, just wait, and the new release will be ready in due time.
tlmgr: package repository ftp://tug.org/historic/systems/texlive/2017/tlnet-final (verified)
tlmgr: no self-updates for tlmgr available.
和
~$ tlmgr install nicefrac
(running on Debian, switching to user mode!)
TeX Live 2017 is frozen forever and will no
longer be updated. This happens in preparation for a new release.
If you're interested in helping to pretest the new release (when
pretests are available), please read http://tug.org/texlive/pretest.html.
Otherwise, just wait, and the new release will be ready in due time.
tlmgr: package repository ftp://tug.org/historic/systems/texlive/2017/tlnet-final (verified)
tlmgr install: package nicefrac not present in repository.
tlmgr: action install returned an error; continuing.
tlmgr: An error has occurred. See above messages. Exiting.
如何安装 2020 版本的 TeX 或如何正确使用 tlmgr 而不会出现这些错误？