Fix test that assumes CARGO_CFG_TARGET_FAMILY is a single value (#16079)

### What does this PR try to resolve?

This test would break if/when `rustc` outputs further `target_family`
cfgs.

An example of this could be `target_family = "linux"`, or `target_family
= "darwin"` if we decide move forwards with
rust-lang/rust#100343.

### How to test and review this PR?

```sh
cargo test -- cfg_env_vars_available
```

Author: epage

tests/testsuite/build_script.rs

@@ -4448,9 +4448,9 @@ fn cfg_env_vars_available() {
                 fn main() {
                     let fam = env::var("CARGO_CFG_TARGET_FAMILY").unwrap();
                     if cfg!(unix) {
-                        assert_eq!(fam, "unix");
-                    } else {
-                        assert_eq!(fam, "windows");
+                        assert!(fam.contains("unix"));
+                    } else if cfg!(windows) {
+                        assert!(fam.contains("windows"));
                     }
                 }
             "#,