ci(nix): don't build for intel macs

These are being dropped by Nixpkgs itself soon in the near future, with
few users (on top of reduced usage of our flake/cache). We also already
have coverage for macOS builds through the aarch64-darwin target, so
this doesn't have a big impact on our end either

Obviously can be reverted if enough people want it

Signed-off-by: Seth Flynn <getchoo@tuta.io>

.github/workflows/nix.yml

@@ -86,9 +86,6 @@ jobs:
           - os: ubuntu-22.04-arm
             system: aarch64-linux
 
-          - os: macos-15-intel
-            system: x86_64-darwin
-
           - os: macos-14
             system: aarch64-darwin