nix: expose sched_priority

This was missed when implementing the corresponding option in hst.

Signed-off-by: Ophestra <cat@gensokyo.uk>

options.nix

@@ -253,6 +253,13 @@ in
                   The zero value retains the current scheduling policy.
                 '';
               };
+              schedPriority = mkOption {
+                type = nullOr (ints.between 1 99);
+                default = null;
+                description = ''
+                  Scheduling priority to set for the container.
+                '';
+              };
 
               nix = mkEnableOption "nix daemon access";
               mapRealUid = mkEnableOption "mapping to priv-user uid";