Rollup merge of #123378 - ferrocene:hoverbear/bump-sysinfo-to-0.30.8, r=Nilstrieb

Update sysinfo to 0.30.8

Fixes a Mac specific issue when using `metrics = true` in `config.toml`.

```config.toml
# Collect information and statistics about the current build and writes it to
# disk. Enabling this or not has no impact on the resulting build output. The
# schema of the file generated by the build metrics feature is unstable, and
# this is not intended to be used during local development.
metrics = true
```

During repeated builds, as the generated `metrics.json` grew, eventually `refresh_cpu()` would be called in quick enough succession (specifically: under 200ms) that a divide by zero would occur, leading to a `NaN` which would not be serialized, then when the `metrics.json` was re-read it would fail to parse.

That error looks like this (collected from Ferrocene's CI):

```
   Compiling rustdoc-tool v0.0.0 (/Users/distiller/project/src/tools/rustdoc)
    Finished release [optimized] target(s) in 38.37s
thread 'main' panicked at src/utils/metrics.rs:180:21:
serde_json::from_slice::<JsonRoot>(&contents) failed with invalid type: null, expected f64 at line 1 column 9598
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Build completed unsuccessfully in 0:00:40

Exited with code exit status 1
```

Related: https://github.com/GuillaumeGomez/sysinfo/pull/1236