nixos/slurm: expose to path config files

This adds a new internal option, services.slurm.etcSlurm such
that other modules can access slurm's config files. This is
needed, for example, when a service wants to run a slurm command.

nixos/modules/services/computing/slurm/slurm.nix

@@ -274,6 +274,15 @@ in
         '';
       };
 
+      etcSlurm = mkOption {
+        type = types.path;
+        internal = true;
+        default = etcSlurm;
+        description = ''
+          Path to directory with slurm config files. This option is set by default from the
+          Slurm module and is meant to make the Slurm config file available to other modules.
+        '';
+      };
 
     };
 
@@ -308,7 +317,7 @@ in
           #!/bin/sh
           if [ -z "$SLURM_CONF" ]
           then
-            SLURM_CONF="${etcSlurm}/slurm.conf" "$EXE" "\$@"
+            SLURM_CONF="${cfg.etcSlurm}/slurm.conf" "$EXE" "\$@"
           else
             "$EXE" "\$0"
           fi