Update releases branch with changes from 19/06 discussion

• set the correct mpsd spack branch
• use gcc in general and not a specific version as system compiler

spack_setup.sh

@@ -7,7 +7,7 @@ set -E -o pipefail
 spack_git='https://github.com/mpsd-computational-science/spack.git'
 spack_git_ssh='git@github.com:mpsd-computational-science/spack.git'
 spack_branch='mpsd/v0.19_dev-23a'
-system_compiler="gcc@10.2.1"
+system_compiler="gcc"
 generic_arch="x86_64_v2"
 # default: spack cache is one level above pwd
 spack_cache=`realpath ../mpsd-spack-cache`