fix: honor environment variables during install (#38974) (#38976)

backport #38974

routers/install/install.go

@@ -312,8 +312,10 @@ func SubmitInstall(ctx *context.Context) {
 		return
 	}
 
-	// Save settings.
+	// Some logic also depends on the config values, so EnvironmentToConfig should also be applied first.
+	// EnvironmentToConfig is applied on each start up, so it also must override the "install form", so it must be applied after (twice).
 	cfg, err := setting.NewConfigProviderFromFile(setting.CustomConf)
+	setting.EnvironmentToConfig(cfg, os.Environ())
 	if err != nil {
 		log.Error("Failed to load custom conf '%s': %v", setting.CustomConf, err)
 	}