adapt installation script to use in CI

install_spack_env.sh

 #!/bin/bash
 
-# ===========================================================================================================
+# ===============================================================================================================================
 # title         : install_spack_env.sh
-# usage         : ./install_spack_env.sh $INSTALLATION_ROOT $SPACK_VERSION $EBRAINS_REPO $EBRAINS_SPACK_ENV
+# usage         : ./install_spack_env.sh $INSTALLATION_ROOT $SPACK_VERSION $EBRAINS_REPO $EBRAINS_SPACK_ENV $UPSTREAM_INSTANCE
 # description   : installs or updates the spack environment defined in the EBRAINS spack repo
 #                 (if the specified spack instance doesn't exist, it also creates it)
-# ===========================================================================================================
+# ===============================================================================================================================
 
 INSTALLATION_ROOT=$1  # where to set up the installation
 SPACK_VERSION=$2      # which spack version to use
 EBRAINS_REPO=$3       # location of ebrains-spack-builds repository
 EBRAINS_SPACK_ENV=$4  # name of EBRAINS Spack environment to be created/updated
+UPSTREAM_INSTANCE=$5  # path to Spack instance to use as upstream (optional)
 
 # specify location of .spack dir (by default in ~)
 # this is where cache and configuration settings are stored
@@ -29,26 +30,34 @@ then
   sed -i "s/self.file_like, \"w\"/self.file_like, \"a\"/g" $INSTALLATION_ROOT/spack/lib/spack/llnl/util/tty/log.py
 fi
 
+if [[ $UPSTREAM_INSTANCE ]]
+then
+  cat <<EOF > $INSTALLATION_ROOT/spack/etc/spack/defaults/upstreams.yaml
+upstreams:
+  upstream-spack-instance:
+    install_tree: $UPSTREAM_INSTANCE/spack/opt/spack
+EOF
+fi
+
 # activate Spack
 source $INSTALLATION_ROOT/spack/share/spack/setup-env.sh
 
+# add repo if it does not exist
+if [[ ! $(spack repo list | grep $EBRAINS_REPO) ]]
+then
+  spack repo add $EBRAINS_REPO
+fi
+
 # install platform compiler (extract version from packages.yaml)
 if [ $SYSTEMNAME == ebrainslab ]
 then
   EBRAINS_SPACK_COMPILER=$(grep 'compiler' $EBRAINS_REPO/site-config/$SYSTEMNAME/packages.yaml | awk -F'[][]' '{ print $2 }')
   spack compiler find
-  spack compiler list | grep -q $EBRAINS_SPACK_COMPILER || spack install $EBRAINS_SPACK_COMPILER
-  spack load $EBRAINS_SPACK_COMPILER
+  spack load $EBRAINS_SPACK_COMPILER || (spack install $EBRAINS_SPACK_COMPILER; spack load $EBRAINS_SPACK_COMPILER)
 fi
 
 spack compiler find
 
-# add repo if it does not exist
-if [[ ! $(spack repo list | grep $EBRAINS_REPO) ]]
-then
-  spack repo add $EBRAINS_REPO
-fi
-
 # create environment if it does not exist
 if [ ! -d "$SPACK_ROOT/var/spack/environments/$EBRAINS_SPACK_ENV" ]
 then