Forbid std::process::Command spawning, replace with smol where appropriate (#38894)

std commands can block for an arbitrary duration and so runs risk of
blocking tasks for too long. This replaces all such uses where sensible
with async processes.

Release Notes:

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

crates/cli/src/main.rs

@@ -1,3 +1,7 @@
+#![allow(
+    clippy::disallowed_methods,
+    reason = "We are not in an async environment, so std::process::Command is fine"
+)]
 #![cfg_attr(
     any(target_os = "linux", target_os = "freebsd", target_os = "windows"),
     allow(dead_code)