Support cache (#25)

See [Caching dependencies to speed up workflows](https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows).

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/25
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Zettat123 <zettat123@noreply.gitea.io>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-committed-by: Jason Song <i@wolfogre.com>

build.go

@@ -0,0 +1,11 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build vendor
+
+package main
+
+import (
+	// for vet
+	_ "code.gitea.io/gitea-vet"
+)