fixup: add target feature flag

Author: not-matthias

crates/cargo-codspeed/tests/cargo_config.in/.cargo/config.toml

@@ -3,5 +3,8 @@
 [build]
 rustflags = ["--cfg", "this_does_nothing"]
 
+[target.x86_64-unknown-linux-gnu]
+rustflags = ["--cfg", "target_feature_flag"]
+
 [target.'cfg(all())']
 rustflags = ["--cfg", "custom_feature_flag"]

crates/cargo-codspeed/tests/cargo_config.in/benches/cargo_config_bench.rs

@@ -8,6 +8,11 @@ fn bench_failing_without_custom_flag(c: &mut Criterion) {
             compile_error!(
                 "custom_feature_flag is not enabled - .cargo/config.toml rustflags not applied"
             );
+
+            #[cfg(not(target_feature_flag))]
+            compile_error!(
+                "target_feature_flag is not enabled - .cargo/config.toml rustflags not applied"
+            );
         })
     });
 }