fix local workflow for act_runner exec (#63)

by the way, export `ACT_SKIP_CHECKOUT` as a env verb for user to do some special config of local test.

example usage:

7a3ab0fdbc

Reviewed-on: https://gitea.com/gitea/act/pulls/63
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: a1012112796 <1012112796@qq.com>
Co-committed-by: a1012112796 <1012112796@qq.com>

act/runner/run_context.go

@@ -82,6 +82,11 @@ func (rc *RunContext) GetEnv() map[string]string {
 		}
 	}
 	rc.Env["ACT"] = "true"
+
+	if !rc.Config.NoSkipCheckout {
+		rc.Env["ACT_SKIP_CHECKOUT"] = "true"
+	}
+
 	return rc.Env
 }