Follow upstream support for variables (#66)

Because the upstream [PR](https://github.com/nektos/act/pull/1833) already supports variables, so this PR revert #43 (commit de529139af), and cherry-pick commit [6ce45e3](6ce45e3f24).

Co-authored-by: Kuan Yong <wong0514@gmail.com>
Reviewed-on: https://gitea.com/gitea/act/pulls/66
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_test.go

@@ -557,6 +557,7 @@ func TestContexts(t *testing.T) {
 		// {"contains(steps.*.outputs.name, 'value')", true, "steps-context-array-outputs"},
 		{"runner.os", "Linux", "runner-context"},
 		{"secrets.name", "value", "secrets-context"},
+		{"vars.name", "value", "vars-context"},
 		{"strategy.fail-fast", true, "strategy-context"},
 		{"matrix.os", "Linux", "matrix-context"},
 		{"needs.job-id.outputs.output-name", "value", "needs-context"},
@@ -593,6 +594,9 @@ func TestContexts(t *testing.T) {
 		Secrets: map[string]string{
 			"name": "value",
 		},
+		Vars: map[string]string{
+			"name": "value",
+		},
 		Strategy: map[string]interface{}{
 			"fail-fast": true,
 		},