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/exprparser/interpreter.go

@@ -23,6 +23,8 @@ type EvaluationEnvironment struct {
 	Matrix   map[string]interface{}
 	Needs    map[string]Needs
 	Inputs   map[string]interface{}
+
+	Vars map[string]string
 }
 
 type Needs struct {
@@ -181,6 +183,8 @@ func (impl *interperterImpl) evaluateVariable(variableNode *actionlint.VariableN
 		return math.Inf(1), nil
 	case "nan":
 		return math.NaN(), nil
+	case "vars":
+		return impl.env.Vars, nil
 	default:
 		return nil, fmt.Errorf("Unavailable context: %s", variableNode.Name)
 	}