Print env var in --print=deployment-target

The deployment target environment variable is OS-specific, and if you're
in a place where you're asking `rustc` for the deployment target, you're
likely to also wanna know the environment variable.

compiler/rustc_driver_impl/src/lib.rs

@@ -875,8 +875,9 @@ fn print_crate_info(
             DeploymentTarget => {
                 if sess.target.is_like_osx {
                     println_info!(
-                        "deployment_target={}",
-                        apple::pretty_version(apple::deployment_target(sess))
+                        "{}={}",
+                        apple::deployment_target_env_var(&sess.target.os),
+                        apple::pretty_version(apple::deployment_target(sess)),
                     )
                 } else {
                     #[allow(rustc::diagnostic_outside_of_impl)]