Migrate all examples to use std::env::var_os (#312)

examples/static-file-server/src/main.rs

@@ -11,7 +11,7 @@ use tower_http::{services::ServeDir, trace::TraceLayer};
 #[tokio::main]
 async fn main() {
     // Set the RUST_LOG, if it hasn't been explicitly defined
-    if std::env::var("RUST_LOG").is_err() {
+    if std::env::var_os("RUST_LOG").is_none() {
         std::env::set_var(
             "RUST_LOG",
             "example_static_file_server=debug,tower_http=debug",