Fix build on *BSD, illumos, etc. (#6456)

* nu-path: use 'linux' code on all non-macOS unix

* nu-command: cfg() the Ps command to platforms it's actually implemented on

* nu-system: cfg() the Ps test to the platforms Ps is implemented on

crates/nu-command/src/default_context.rs

@@ -160,10 +160,17 @@ pub fn create_default_context() -> EngineState {
             Exec,
             External,
             NuCheck,
-            Ps,
             Sys,
         };
 
+        #[cfg(any(
+            target_os = "android",
+            target_os = "linux",
+            target_os = "macos",
+            target_os = "windows"
+        ))]
+        bind_command! { Ps };
+
         #[cfg(feature = "which-support")]
         bind_command! { Which };