[crush] Fix os.execute() failure detection on LuaJIT.

crush.lua

@@ -205,7 +205,11 @@ local function fetch(dep)
         fullcmd = chdir("lib", quiet(cmd))
     end
 
-    if not os.execute(fullcmd) then
+    -- On success, os.execute() returns:
+    -- true on regular Lua
+    -- 0 on LuaJIT (actual OS error code)
+    local code = os.execute(fullcmd)
+    if code ~= true and code ~= 0 then
         error(dep.name..": Dependency fetch failed ("..cmd..").")
     end
 end