Make Gitea always use its internal config, ignore /etc/gitconfig (#33076)

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.

modules/setting/config_env.go

@@ -177,7 +177,10 @@ func InitGiteaEnvVars() {
 	// But git would try "XDG_CONFIG_HOME/git/config" first if "HOME/.gitconfig" does not exist,
 	// then our git.InitFull would still write to "XDG_CONFIG_HOME/git/config" if XDG_CONFIG_HOME is set.
 	_ = os.Unsetenv("XDG_CONFIG_HOME")
+}
 
+func InitGiteaEnvVarsForTesting() {
+	InitGiteaEnvVars()
 	_ = os.Unsetenv("GIT_AUTHOR_NAME")
 	_ = os.Unsetenv("GIT_AUTHOR_EMAIL")
 	_ = os.Unsetenv("GIT_AUTHOR_DATE")