fix: check if env is complete before fetching

spack fetch --missing fails when there are no missing packages

install_spack_env.sh

@@ -80,7 +80,7 @@ spack env activate $EBRAINS_SPACK_ENV
 
 # fetch all sources
 spack concretize --fresh --test root
-spack fetch --dependencies --missing
+spack-python -c "exit(not len(spack.environment.active_environment().uninstalled_specs()))" && spack fetch --dependencies --missing
 rm $SPACK_ROOT/var/spack/environments/$EBRAINS_SPACK_ENV/spack.lock
 
 # install the environment, use 2 jobs to reduce the amount of required RAM