In history, Gitea could use the system config `/etc/gitconfig` because
some users said that "they might put certNonceSeed in it"
Actually, we shouldn't not use it, because it also causes conflicts
(there are already some fixes like #28848)
To make the system clear, I think it's worth to introduce the breaking
change: add `GIT_CONFIG_NOSYSTEM=1` to all git commands.
## ⚠️ BREAKING ⚠️
For most users, nothing need to do.
If you have made changes to `/etc/gitconfig` to affect Gitea's behavior,
you need to move these config options to Gitea's internal git config
file, it is usually in Gitea's `{AppDataPath}/home/.git` directory.
