Only use clone3 when needed for pidfd

In rust-lang#89522 we learned that `clone3` is interacting poorly with Gentoo's
`sandbox` tool. We only need that for the unstable pidfd extensions, so
otherwise avoid that and use a normal `fork`.

library/std/src/sys/unix/process/process_unix.rs

@@ -166,21 +166,20 @@ impl Command {
             fn clone3(cl_args: *mut clone_args, len: libc::size_t) -> libc::c_long
         }
 
+        // Bypassing libc for `clone3` can make further libc calls unsafe,
+        // so we use it sparingly for now. See #89522 for details.
+        let want_clone3_pidfd = self.get_create_pidfd();
+
         // If we fail to create a pidfd for any reason, this will
         // stay as -1, which indicates an error.
         let mut pidfd: pid_t = -1;
 
         // Attempt to use the `clone3` syscall, which supports more arguments
         // (in particular, the ability to create a pidfd). If this fails,
         // we will fall through this block to a call to `fork()`
-        if HAS_CLONE3.load(Ordering::Relaxed) {
-            let mut flags = 0;
-            if self.get_create_pidfd() {
-                flags |= CLONE_PIDFD;
-            }
-
+        if want_clone3_pidfd && HAS_CLONE3.load(Ordering::Relaxed) {
             let mut args = clone_args {
-                flags,
+                flags: CLONE_PIDFD,
                 pidfd: &mut pidfd as *mut pid_t as u64,
                 child_tid: 0,
                 parent_tid: 0,