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.

spack-environment/packages.yaml

@@ -163,7 +163,7 @@ packages:
     - -ipo +podio +root +zmq
   juggler:
     require:
-    - '@11.0.0'
+    - '@12.0.0'
     - cxxstd=20
   k4fwcore:
     require: