fix: spack:config:environments_root:~/.spack/env

• Avoids users getting stuck with unwritable /opt/spack/var/spack/environments when creating new environments.
• Accept unsigned buildcache packages, as for those of build dependencies that are garbage collected away (not sure they are relocatable, but that's a problem for later).

containers/jug/dev.Dockerfile

@@ -241,6 +241,7 @@ RUN . /opt/spack/share/spack/setup-env.sh                               \
  && spack config --scope site add "config:install_tree:root:~/spack"    \
  && spack config --scope site add "config:source_cache:~/.spack/cache"  \
  && spack config --scope site add "config:binary_index_root:~/.spack"   \
+ && spack config --scope site add "config:environments_root:~/.spack/env" \
  && spack config blame config                                           \
  && spack config --scope site add "upstreams:eic-shell:install_tree:/opt/software" \
  && spack config blame upstreams