Do not prepare oauth2 config if it is not enabled, do not write config in some sub-commands (#25567) (#25576)

Backport #25567

Ref:

* https://github.com/go-gitea/gitea/issues/25377#issuecomment-1609757289

And some sub-commands like "generate" / "docs", they do not need to use
the ini config

modules/setting/oauth2.go

@@ -116,6 +116,10 @@ func loadOAuth2From(rootCfg ConfigProvider) {
 		return
 	}
 
+	if !OAuth2.Enable {
+		return
+	}
+
 	if !filepath.IsAbs(OAuth2.JWTSigningPrivateKeyFile) {
 		OAuth2.JWTSigningPrivateKeyFile = filepath.Join(AppDataPath, OAuth2.JWTSigningPrivateKeyFile)
 	}