Support gitea context (#27)

And we will be able to use context like `${{ gitea.repository }}` in workflows yaml files, it's same as `${{ github.repository }}`

Reviewed-on: https://gitea.com/gitea/act/pulls/27
Reviewed-by: Zettat123 <[email protected]>

pkg/exprparser/interpreter.go

@@ -152,6 +152,8 @@ func (impl *interperterImpl) evaluateVariable(variableNode *actionlint.VariableN
 	switch strings.ToLower(variableNode.Name) {
 	case "github":
 		return impl.env.Github, nil
+	case "gitea": // compatible with Gitea
+		return impl.env.Github, nil
 	case "env":
 		return impl.env.Env, nil
 	case "job":