clippy: simplify code according to nightly 'manual_unwrap_or_default' lint

https://rust-lang.github.io/rust-clippy/master/index.html#/manual_unwrap_or_default

tests/by-util/test_pinky.rs

@@ -54,10 +54,7 @@ fn test_long_format_multiple_users() {
     // multiple instances of one account we know exists,
     // the account of the test runner,
     // and an account that (probably) doesn't exist
-    let runner = match std::env::var("USER") {
-        Ok(user) => user,
-        Err(_) => String::new(),
-    };
+    let runner = std::env::var("USER").unwrap_or_default();
     let args = ["-l", "root", "root", "root", &runner, "no_such_user"];
     let ts = TestScenario::new(util_name!());
     let expect = unwrap_or_return!(expected_result(&ts, &args));