ditch macos-14

To save some energy, it's very unlikely to fail if macos-15 passes.

Author: Anton-4

.github/workflows/ci_nix.yml

@@ -16,7 +16,7 @@ jobs:
       fail-fast: false
       matrix:
         # macos-13 uses x86-64 machine, macos-14 & 15 use aarch64
-        os: [macos-13, macos-14, macos-15, ubuntu-22.04, ubuntu-24.04-arm]
+        os: [macos-13, macos-15, ubuntu-22.04, ubuntu-24.04-arm]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4