Explicitly enable MT for geant4.

spack.yaml

@@ -23,7 +23,7 @@ spack:
     - stow@2.3.1
     - cairo@1.16.0 +fc+ft+X+pdf+gobject
     - podio@0.13
-    - geant4@10.7.1 cxxstd=17 +opengl +vecgeom +x11 +qt ^qt +opengl
+    - geant4@10.7.1 cxxstd=17 +opengl +vecgeom +x11 +qt +threads ^qt +opengl
     - dd4hep@1.16.1 +geant4 +assimp +hepmc3 +ipo
     - acts@8.02.0p1 +dd4hep +digitization +identification +json +tgeo +ipo
     - genfit@2.00.00