feat: install most dependencies in cacheable layer, before adding main

Instead of requiring the whole stack to be reinstalled whenever a specific version (e.g. main) causes a cache fail, this aims to have stable default version with a second-stage installation of the potentially-modified versions of eicrecon and juggler. This should allow all builds to cache the first installation in virtually all cases, leading to faster builds.

containers/jug/dev.Dockerfile

@@ -163,10 +163,10 @@ spack buildcache update-index local
 spack buildcache update-index eics3rw
 spack env activate --dir ${SPACK_ENV}
 if [ -n ${JUGGLER_VERSION} ] ; then
-  spack rm juggler && spack add juggler@git.${JUGGLER_VERSION}
+  spack config add "packages:juggler:require:['@git.${JUGGLER_VERSION}']"
 fi
 if [ -n ${EICRECON_VERSION} ] ; then
-  spack rm eicrecon && spack add eicrecon@git.${EICRECON_VERSION}
+  spack config add "packages:eicrecon:require:['@git.${EICRECON_VERSION}']"
 fi
 spack concretize --fresh --force --quiet
 make --jobs ${jobs} --keep-going --directory /opt/spack-environment SPACK_ENV=${SPACK_ENV} BUILDCACHE_MIRROR="local eics3rw"