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/dev/spack.yaml

@@ -77,6 +77,7 @@ spack:
     - py-wurlitzer
     - py-yapf
     - root
+    - snakemake
     - spdlog
     - stow
     - tensorflow-lite