Explicitly set tokio flags

`rt-multi-thread` is required for Windows, and we rely on inheriting it
from `reqwest`. This sets it explicitly.

xtask/Cargo.toml

@@ -31,7 +31,7 @@ serde = { workspace = true, features = ["derive"] }
 serde_json_traversal = { workspace = true }
 shell-candy = { workspace = true }
 tempfile = { workspace = true }
-tokio = { workspace = true, features = ["rt"] }
+tokio = { workspace = true, features = ["rt", "rt-multi-thread"] }
 tokio-stream = { workspace = true }
 uuid = { workspace = true, features = ["v4"] }
 which = { workspace = true }