feat(KeySetCache): use a artifacts to store the cache

compiler/lib/ClientLib/KeySetCache.cpp

@@ -13,6 +13,7 @@
 #include <fstream>
 #include <sstream>
 #include <string>
+#include <utime.h>
 
 extern "C" {
 #include "concrete-ffi.h"
@@ -78,6 +79,10 @@ KeySetCache::loadKeys(ClientParameters &params, uint64_t seed_msb,
   // TODO: text dump of all parameter in /hash
   auto key_set = std::make_unique<KeySet>();
 
+  // Mark the folder as recently use.
+  // e.g. so the CI can do some cleanup of unused keys.
+  utime(folderPath.c_str(), nullptr);
+
   std::map<LweSecretKeyID, std::pair<LweSecretKeyParam, LweSecretKey_u64 *>>
       secretKeys;
   std::map<LweSecretKeyID, std::pair<BootstrapKeyParam, LweBootstrapKey_u64 *>>