use build tag to support darwin again (#600)

fix https://ci.woodpecker-ci.org/woodpecker-ci/woodpecker/build/3344/11

Reviewed-on: https://gitea.com/gitea/go-sdk/pulls/600
Reviewed-by: John Olheiser <john+gitea@jolheiser.com>
Reviewed-by: techknowlogick <techknowlogick@gitea.io>

gitea/agent.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a MIT-style
 // license that can be found in the LICENSE file.
 
+//go:build !windows
+
 package gitea
 
 import (

gitea/agent_darwin.go

@@ -1 +0,0 @@
-agent_linux.go

gitea/agent_windows.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a MIT-style
 // license that can be found in the LICENSE file.
 
+//go:build windows
+
 package gitea
 
 import (