Pre-initialize global rayon threadpool (#41226)

We only use it a handful of times and the default amount of threads
(logical cpu core number) its spawns is overkill for this. This also
gives the threads names oppose to being labeled `<unknown>`

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/cli/Cargo.toml

@@ -32,6 +32,7 @@ release_channel.workspace = true
 serde.workspace = true
 util.workspace = true
 tempfile.workspace = true
+rayon.workspace = true
 
 [target.'cfg(any(target_os = "linux", target_os = "freebsd"))'.dependencies]
 exec.workspace = true