Support configuration variables (#43)

related to: https://gitea.com/gitea/act_runner/issues/127

This PR make `act` support the expression like `${{ vars.YOUR_CUSTOM_VARIABLES }}`.

Reviewed-on: https://gitea.com/gitea/act/pulls/43
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: sillyguodong <gedong_1994@163.com>
Co-committed-by: sillyguodong <gedong_1994@163.com>

pkg/jobparser/interpeter.go

@@ -62,6 +62,8 @@ func NewInterpeter(
 		Matrix:   matrix,
 		Needs:    using,
 		Inputs:   nil, // not supported yet
+
+		Vars: nil,
 	}
 
 	config := exprparser.Config{