dnc: fix systemd service

Remove the needs of dnc user. We should not force admins to create
a new dedicated user.
Use a env variable to cache data and add fallbacks to avoid any crash

Change-Id: If8fa2ced856c36f2d63870f9b6a6fc7839d91040
diff --git a/tools/dsh/dsh.cpp b/tools/dsh/dsh.cpp
index 9da0544..8456612 100644
--- a/tools/dsh/dsh.cpp
+++ b/tools/dsh/dsh.cpp
@@ -95,7 +95,7 @@
     :logger(dht::log::getStdLogger())
     , ioContext(std::make_shared<asio::io_context>()),
     iceFactory(std::make_shared<IceTransportFactory>(logger)),
-    certStore(std::make_shared<tls::CertificateStore>(PATH/"certstore", logger)),
+    certStore(std::make_shared<tls::CertificateStore>(cachePath()/"certstore", logger)),
     trustStore(std::make_shared<tls::TrustStore>(*certStore))
 {
     ioContext = std::make_shared<asio::io_context>();