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.

eic-spack.sh

@@ -3,7 +3,7 @@ EICSPACK_ORGREPO="eic/eic-spack"
 
 ## EIC spack commit hash or github version, e.g. v0.19.7
 ## note: nightly builds could use a branch e.g. releases/v0.19
-EICSPACK_VERSION="1b6a01ce9081c9c1476975b90d8348e5a3522cd1"
+EICSPACK_VERSION="6a708734d75d9061cb66919eb52a0a9cfc8927ca"
 
 ## Space-separated list of eic-spack cherry-picks
 read -r -d '' EICSPACK_CHERRYPICKS <<- \