zynq: set OPENOCD_ZYNQ

zynq.nix

@@ -53,6 +53,7 @@ in
         echo Power cycle done.
 
         export USER=hydra
+        export OPENOCD_ZYNQ=${<zynq-rs>}/openocd
         pushd ${<artiq-zynq>}
         bash ${<artiq-zynq>}/remote_run.sh -h rpi-4 -o "-F /dev/null -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o LogLevel=ERROR -i /opt/hydra_id_rsa" -d ${artiq-zynq.zc706-nist_qc2-jtag}
         popd