Ignore clippy::unnecessary_map_or lint

```
error: this `map_or` is redundant
  --> tests/no-core/build.rs:15:5
   |
15 | /     env::var_os("RUSTC")
16 | |         .and_then(|rustc| Command::new(rustc).arg("--version").output().ok())
17 | |         .and_then(|output| String::from_utf8(output.stdout).ok())
18 | |         .map_or(false, |version| version.contains("nightly") || version.contains("dev"))
   | |________________________________________________________________________________________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_map_or
   = note: `-D clippy::unnecessary-map-or` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_map_or)]`
help: use is_some_and instead
   |
15 ~     env::var_os("RUSTC")
16 +         .and_then(|rustc| Command::new(rustc).arg("--version").output().ok())
17 +         .and_then(|output| String::from_utf8(output.stdout).ok()).is_some_and(|version| version.contains("nightly") || version.contains("dev"))
   |
```

tests/no-core/Cargo.toml

@@ -2,6 +2,7 @@
 name = "no-core"
 version = "0.0.0"
 edition = "2018"
+rust-version = "1.37" # Prevent clippy from suggesting a code that requires a new version.
 publish = false
 
 [lib]