fedora-remote: set GRD credentials with explicit arguments

grdctl's interactive set-credentials reads from a controlling terminal, which a
systemd service does not have, so piping the username/password in set nothing
and GRD denied every client with 'Credentials are not set'. Pass them as
arguments instead.

images/fedora-remote/files/grd-firstboot

@@ -27,9 +27,12 @@ fi
 grdctl --system rdp set-tls-key "${TLS_DIR}/tls.key"
 grdctl --system rdp set-tls-cert "${TLS_DIR}/tls.crt"
 
+# Pass username/password as arguments: grdctl's interactive prompt reads from a
+# controlling terminal, which a system service does not have, so piping them in
+# silently sets nothing (GRD then logs "Credentials are not set, denying client").
 if [[ -n "${GRD_SYSTEM_PASSWORD:-}" ]]; then
-  printf '%s\n%s\n' "${GRD_SYSTEM_USER:-rdp}" "${GRD_SYSTEM_PASSWORD}" \
-    | grdctl --system rdp set-credentials
+  grdctl --system rdp set-credentials \
+    "${GRD_SYSTEM_USER:-rdp}" "${GRD_SYSTEM_PASSWORD}"
 fi
 
 grdctl --system rdp enable