make env/var test deterministic

src/tools/miri/tests/pass/shims/env/var.rs

@@ -1,3 +1,4 @@
+//@compile-flags: -Zmiri-preemption-rate=0
 use std::env;
 use std::thread;
 
@@ -26,6 +27,8 @@ fn main() {
     println!("{:#?}", env::vars().collect::<Vec<_>>());
 
     // Do things concurrently, to make sure there's no data race.
+    // We disable preemption to make sure the lock is not contended;
+    // that means we don't hit e.g. the futex codepath on Android (which we don't support).
     let t = thread::spawn(|| {
         env::set_var("MIRI_TEST", "42");
     });