feat: celeritas: require +root

Avoid errors like:

jug_dev> wdconinc@menelaos:~/git/epic$ celer-export-geant epic.gdml '' output.json
info: Using default Celeritas Geant4 options
status: Initializing Geant4 run manager
status: Initializing Geant4 geometry
info: Loading Geant4 geometry from GDML at epic.gdml
status: Building Geant4 physics tables
status: Transferring data from Geant4
status: Loading external elemental data
info: Opening JSON output at output.json
/tmp/root/spack-stage/spack-stage-celeritas-0.5.0-luyvi73exkq3jx55muwzls2cmatui6dc/spack-src/app/celer-export-geant.cc:193: critical: Runtime error: celeritas: configuration error: required dependency is disabled in this build: ROOT

spack-environment/packages.yaml

@@ -49,6 +49,10 @@ packages:
     require:
     - '@3.4.0'
     - -ipo
+  celeritas:
+    require:
+    - '@0.5.0'
+    - +geant4 +hepmc3 +root +vecgeom
   cernlib:
     require:
     - '@2022.11.08.0-free'