Do not validate UTF-8 of RUST_LOG twice (rust-lang#323)

Another one is in `env_logger::init()`

Author: tesuji

blacksmith/src/main.rs

@@ -13,8 +13,8 @@ const CACHE_TTL_SECONDS: u64 = 3600; // 1 hour
 
 fn main() {
     // If RUST_LOG is present use that, else default to info level printing.
-    if env::var("RUST_LOG").is_ok() {
-        env_logger::init();
+    if let Some(v) = env::var("RUST_LOG") {
+        env_logger::init_from_env(v);
     } else {
         env_logger::builder()
             .filter_level(log::LevelFilter::Info)