gnu: stockfish: Factor out the neural network revision.

* gnu/packages/games.scm (stockfish): Factor out the neural-network-revision.

gnu/packages/games.scm

@@ -11962,6 +11962,7 @@ (define-public chessx
     (license license:gpl2+)))
 
 (define-public stockfish
+  (let ((neural-network-revision "3475407dc199")) ; also update hash below
     (package
       (name "stockfish")
       (version "14")
@@ -11979,9 +11980,11 @@ (define-public stockfish
        `(("neural-network"
           ,(origin
              (method url-fetch)
-           (uri "https://tests.stockfishchess.org/api/nn/nn-3475407dc199.nnue")
+             (uri (string-append "https://tests.stockfishchess.org/api/nn/nn-"
+                                 neural-network-revision ".nnue"))
              (sha256
-            (base32 "11zci5kgwdw9rh8w2w4p84764g82rr666y3n8r2flwwrq5yl0x9l"))))))
+              (base32
+               "11zci5kgwdw9rh8w2w4p84764g82rr666y3n8r2flwwrq5yl0x9l"))))))
       (arguments
        `(#:tests? #f
          #:make-flags (list "-C" "src"
@@ -12003,15 +12006,15 @@ (define-public stockfish
                     (add-after 'unpack 'copy-net
                       (lambda* (#:key inputs #:allow-other-keys)
                         (copy-file (assoc-ref inputs "neural-network")
-                                 "src/nn-3475407dc199.nnue")
-                      #t)))))
+                                   (format #f "src/nn-~a.nnue"
+                                           ,neural-network-revision)))))))
       (synopsis "Strong chess engine")
       (description
        "Stockfish is a very strong chess engine.  It is much stronger than the
 best human chess grandmasters.  It can be used with UCI-compatible GUIs like
 ChessX.")
       (home-page "https://stockfishchess.org/")
-    (license license:gpl3+)))
+      (license license:gpl3+))))
 
 (define-public barrage
   (package