Write absolute AppDataPath to app.ini when installing (go-gitea#25331)

If the APP_DATA_PATH isn't written into the config when installing, then
its value is uncertain because some Gitea command doesn't run with
correct WorkPath.

This is a quick fix for go-gitea#25330  and can be backported.

routers/install/install.go

@@ -393,6 +393,7 @@ func SubmitInstall(ctx *context.Context) {
 	cfg.Section("server").Key("DOMAIN").SetValue(form.Domain)
 	cfg.Section("server").Key("HTTP_PORT").SetValue(form.HTTPPort)
 	cfg.Section("server").Key("ROOT_URL").SetValue(form.AppURL)
+	cfg.Section("server").Key("APP_DATA_PATH").SetValue(setting.AppDataPath)
 
 	if form.SSHPort == 0 {
 		cfg.Section("server").Key("DISABLE_SSH").SetValue("true")