lammps: set LAMMPS_POTENTIALS in the runtime environment (#16643)

so lammps can find the installed potential files

var/spack/repos/builtin/packages/lammps/package.py

@@ -182,3 +182,7 @@ def cmake_args(self):
             args.append('-DEXTERNAL_KOKKOS=ON')
 
         return args
+
+    def setup_run_environment(self, env):
+        env.set('LAMMPS_POTENTIALS',
+                self.prefix.share.lammps.potentials)