[delphes] align env var with lcg release (#18080)

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

@@ -62,4 +62,4 @@ def cmake_args(self):
 
     def setup_run_environment(self, env):
         # make the cards distributed with delphes more easily accessible
-        env.set('DELPHESCARDS', self.prefix.cards)
+        env.set('DELPHES_DIR', self.prefix)