fix link in spack instructions

Author: vvolkl

spack-build-instructions/README.md

@@ -19,7 +19,10 @@ Using the spack commands can give a more fine-grained control over loaded packag
 On centos7 machines with cvmfs, the easiest way to set up spack is to download a pre-configured instance that is created automatically from the current key4hep/key4hep-spack repository.
 
 ```bash
-curl -L -o spack.tar.gz https://gitlab.cern.ch/key4hep/k4-deploy/-/jobs/artifacts/master/raw/key4hep-spack_centos7-cvmfs.tar.gz?job=build-spack-nightlies
+# download spack artifact created by ci --
+# pre-configured for key4hep use on lxplus/centos7
+
+curl -L -o spack.tar.gz https://gitlab.cern.ch/key4hep/k4-deploy/-/jobs/artifacts/release/raw/key4hep-spack_centos7-cvmfs.tar.gz?job=setup_spack_push
 tar xfz spack.tar.gz && rm spack.tar.gz
 source spack/share/spack/setup-env.sh
 ```