remove or update 'ignore-windows' annotations in some tests

tests/compile-fail/environ-gets-deallocated.rs

@@ -1,4 +1,4 @@
-//ignore-windows: TODO env var emulation stubbed out on Windows
+//ignore-windows: Windows does not have a global environ list that the program can access directly
 
 #[cfg(target_os="linux")]
 fn get_environ() -> *const *const u8 {

tests/run-pass/env-exclude.rs

@@ -1,4 +1,3 @@
-// ignore-windows: TODO env var emulation stubbed out on Windows
 // compile-flags: -Zmiri-disable-isolation -Zmiri-env-exclude=MIRI_ENV_VAR_TEST
 
 fn main() {

tests/run-pass/env-without-isolation.rs

@@ -1,4 +1,3 @@
-// ignore-windows: TODO env var emulation stubbed out on Windows
 // compile-flags: -Zmiri-disable-isolation
 
 fn main() {