ci: remove heavy nix-desktop workflow

.github/workflows/nix-desktop.yml

@@ -1,37 +0,0 @@
-name: nix-desktop
-
-on:
-  pull_request:
-    paths:
-      - "flake.nix"
-      - "flake.lock"
-      - "nix/**"
-      - "packages/app/**"
-      - "packages/desktop/**"
-      - ".github/workflows/nix-desktop.yml"
-  workflow_dispatch:
-
-jobs:
-  nix-desktop:
-    strategy:
-      fail-fast: false
-      matrix:
-        os:
-          - blacksmith-4vcpu-ubuntu-2404
-          - blacksmith-4vcpu-ubuntu-2404-arm
-          - macos-15-intel
-          - macos-latest
-    runs-on: ${{ matrix.os }}
-    timeout-minutes: 60
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v6
-
-      - name: Setup Nix
-        uses: nixbuild/nix-quick-install-action@v34
-
-      - name: Build desktop via flake
-        run: |
-          set -euo pipefail
-          nix --version
-          nix build .#desktop -L