Fix CPU usage info in sys (#8321)

Closes #8264. This PR does a few things to fix the `usage` column in
`sys.cpu`:

1. Sleep a while (~400ms) between calls to `sys.refresh_cpu()`, [as
required by
`sysinfo`](https://docs.rs/sysinfo/latest/sysinfo/trait.SystemExt.html#method.refresh_cpu)
2. Change `sys` to return a `LazyRecord` (so you can do things like `sys
| get host` instantly without waiting for CPU info)
3. Update our `sysinfo` dependency to [fix CPU usage calculations on
Linux](https://github.com/GuillaumeGomez/sysinfo/pull/946)

CPU usage is no longer always reported as zero:


![image](https://user-images.githubusercontent.com/26268125/222929775-5e9cbe18-95d9-4ecb-baf8-1e843f5c7086.png)

crates/nu-cli/Cargo.toml

@@ -36,7 +36,7 @@ once_cell = "1.17.0"
 log = "0.4"
 miette = { version = "5.5.0", features = ["fancy-no-backtrace"] }
 percent-encoding = "2"
-sysinfo = "0.28.0"
+sysinfo = "0.28.2"
 thiserror = "1.0.31"
 
 [features]