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 ...

Cargo.lock

@@ -3073,6 +3073,7 @@ dependencies = [
  "parking_lot",
  "paths",
  "plist",
+ "rayon",
  "release_channel",
  "serde",
  "tempfile",
@@ -13890,6 +13891,7 @@ dependencies = [
  "pretty_assertions",
  "project",
  "proto",
+ "rayon",
  "release_channel",
  "remote",
  "reqwest_client",