feat(xtask): add unused tag for `export_env`

Signed-off-by: Woshiluo Luo <woshiluo.luo@outlook.com>

xtask/src/utils/envs.rs

@@ -1,3 +1,4 @@
+#[allow(unused)]
 macro_rules! export_env {
     ($env:literal ?= $val:expr) => {
         if std::env::vars_os().all(|(k, _)| k != $env) {