Sysinfo plugin does not compile without `multi_threaded` feature

[Friz64]

Bevy version

Any version after #13693

What you did

Compiled bevy without the multi_threaded feature, but with the sysinfo_plugin feature

What went wrong

error[E0308]: mismatched types
    --> crates/bevy_diagnostic/src/system_information_diagnostics_plugin.rs:147:30
     |
147  |             tasks.tasks.push(task);
     |                         ---- ^^^^ expected `Task<SysinfoRefreshData>`, found `FakeTask`
     |                         |
     |                         arguments to this method are incorrect
     |
     = note: expected struct `Task<SysinfoRefreshData>`
                found struct `FakeTask`
note: method defined here
    --> /home/friz64/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:1993:12
     |
1993 |     pub fn push(&mut self, value: T) {
     |            ^^^^

[bugsweeper]

That because single_threaded_task_pool::TaskPool::spawn returns FakeTask which can't contain future result, because it's just empty struct, but actualy for non wasm (for wasm it indeed can't do better, because uses wasm spawn method which allows only futures which outputs only empty tuple) it spawns future, gets it's task, waits while executor finishes, and return nothing (FakeTask), throwing away _task with result! I think should be two spawn implementations for wasm (returning empty FakeTask) and for non-wasm which would return ready result T.