Rollup merge of rust-lang#131833 - c-ryan747:patch-1, r=Noratrieb

Add `must_use` to `CommandExt::exec`

[CommandExt::exec](https://fburl.com/0qhpo7nu) returns a `std::io::Error` in the case exec fails, but its not currently marked as `must_use` making it easy to accidentally ignore it.

This PR adds the `must_use` attributed here as i think it fits the definition in the guide of [When to add #[must_use]](https://std-dev-guide.rust-lang.org/policy/must-use.html#when-to-add-must_use)

std/src/os/unix/process.rs

@@ -154,6 +154,7 @@ pub trait CommandExt: Sealed {
     /// required to gracefully handle errors it is recommended to use the
     /// cross-platform `spawn` instead.
     #[stable(feature = "process_exec2", since = "1.9.0")]
+    #[must_use]
     fn exec(&mut self) -> io::Error;
 
     /// Set executable argument