test: fix clippy warnings

```
error: this assertion has a constant value
 --> crates/build-rs-test-lib/src/lib.rs:3:5
  |
3 |     assert!(cfg!(did_run_build_script));
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: consider moving this into a const block: `const { assert!(..) }`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.92.0/index.html#assertions_on_constants
  = note: `-D clippy::assertions-on-constants` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(clippy::assertions_on_constants)]`
```

A bit annoying we can't have `lint.workspace = true` for this crate yet

Author: weihanglo

crates/build-rs-test-lib/src/lib.rs

@@ -1,4 +1,4 @@
 #[test]
 fn test() {
-    assert!(cfg!(did_run_build_script));
+    const { assert!(cfg!(did_run_build_script)) };
 }