ci: nix-flake-update: new token with pr permissions (#4879)

* ci: nix-flake-update: new token with pr permissions

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

.github/workflows/nix-flake-update.yml

@@ -19,4 +19,4 @@ jobs:
           pr-labels: |
             nix
           pr-reviewers: philiptaron,SomeoneSerge
-          token: ${{ secrets.GITHUB_TOKEN }}
+          token: ${{ secrets.FLAKE_TOKEN }}