Using a fixed release of spack instead the development branch.

conf/common/build.yml

@@ -2,7 +2,7 @@ spack:
   init: ""                                                # command to load spack environment, e.g. module load spack,
                                                           # use spack/setup-env.sh if empty
   url: https://github.com/spack/spack.git                 # url to download spack if necessary
-  branch: develop                                         # if downloaded, branch name to use
+  branch: v0.20.1                                          # if downloaded, branch name to use
   externals: ""                                           # list of packages we try to find with spack external find
                                                           # In LMU we need to include slurm to this list.
   compiler: "gcc@12.2.0"                                  # desired compiler for spack