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

@@ -32,7 +32,7 @@ packages:
     - +root +zlib
   algorithms:
     require:
-    - '@main'
+    - '@git.ebc8f85709bc7233574843b843ee0191a5bf3c1d'
   cairo:
     require:
     - '@1.16.0'