chore: Use const init for Mutex

[Expyron]

No description provided.

[gdesmott]

Thanks for your patch.

I think LazyLock would be a better fit here but that requires Rust 1.80 while our MSRV is 1.78.

[Expyron]

There's no need for LazyLock here.
Mutex::new was made const in Rust 1.63, so lazy initialization is no longer necessary.

[gdesmott]

@sdroege : what's the rationale for using LazyLock in gst then?