ci: temporarily disable testing on macOS via Nix

For some reason the Rust example does not work on macOS using
a Nix environment. Not sure why as it works for everything other
environment we test. Will investigate later.

The logs are:
     Updating git repository `https://github.com/Ivan-Velickovic/rust-seL4`
    Updating crates.io index
warning: spurious network error (3 tries remaining): [35] SSL connect error (OpenSSL/3.0.12: error:16000069:STORE routines::unregistered scheme)
warning: spurious network error (2 tries remaining): [35] SSL connect error (OpenSSL/3.0.12: error:16000069:STORE routines::unregistered scheme)
warning: spurious network error (1 tries remaining): [35] SSL connect error (OpenSSL/3.0.12: error:16000069:STORE routines::unregistered scheme)
error: failed to get `cfg-if` as a dependency of package `sel4-microkit v0.1.0 (https://github.com/Ivan-Velickovic/rust-seL4?branch=dev#e4bde410)`
make: Leaving directory '/Users/runner/work/libvmm/libvmm/examples/rust'
    ... which satisfies git dependency `sel4-microkit` (locked to 0.1.0) of package `vmm v0.1.0 (/Users/runner/work/libvmm/libvmm/examples/rust)`

Caused by:
  failed to query replaced source registry `crates-io`

.github/workflows/ci.yaml

@@ -111,28 +111,28 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           path: build_*/loader.img
-  build_macos_x86_64_nix:
-    name: Build and run examples (macOS x86-64 via Nix)
-    runs-on: macos-12
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-        with:
-          submodules: 'true'
-      - name: Download Microkit SDK
-        run: ./ci/acquire_sdk.sh microkit-sdk.zip ${{ secrets.GITHUB_TOKEN }} macos-x86-64
-        shell: bash
-      - name: Extract Microkit SDK
-        run: unzip microkit-sdk.zip && tar -xf microkit-sdk-1.2.6.tar.gz
-      - name: Install Nix
-        uses: cachix/install-nix-action@v25
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-      - name: Update Nix channel
-        run: nix-channel --update
-      - name: Build and run examples
-        run: nix-shell --pure --run "./ci/examples.sh ${PWD}/microkit-sdk-1.2.6"
-      - name: Upload built system images
-        uses: actions/upload-artifact@v4
-        with:
-          path: build_*/loader.img
+  # build_macos_x86_64_nix:
+  #   name: Build and run examples (macOS x86-64 via Nix)
+  #   runs-on: macos-12
+  #   steps:
+  #     - name: Checkout repository
+  #       uses: actions/checkout@v4
+  #       with:
+  #         submodules: 'true'
+  #     - name: Download Microkit SDK
+  #       run: ./ci/acquire_sdk.sh microkit-sdk.zip ${{ secrets.GITHUB_TOKEN }} macos-x86-64
+  #       shell: bash
+  #     - name: Extract Microkit SDK
+  #       run: unzip microkit-sdk.zip && tar -xf microkit-sdk-1.2.6.tar.gz
+  #     - name: Install Nix
+  #       uses: cachix/install-nix-action@v25
+  #       with:
+  #         nix_path: nixpkgs=channel:nixos-unstable
+  #     - name: Update Nix channel
+  #       run: nix-channel --update
+  #     - name: Build and run examples
+  #       run: nix-shell --pure --run "./ci/examples.sh ${PWD}/microkit-sdk-1.2.6"
+  #     - name: Upload built system images
+  #       uses: actions/upload-artifact@v4
+  #       with:
+  #         path: build_*/loader.img