Explain why rust needs installing Windows 11 arm ci job, but not Windows x886 job

.github/actions/install-deps/action.yml

@@ -16,6 +16,8 @@ runs:
       run: choco install ccache ninja
       if: startsWith(matrix.os, 'windows')
       shell: bash
+      # Windows arm runners don't come with rust by default (see https://github.com/actions/partner-runner-images/blob/main/images/arm-windows-11-image.md)
+      # but the x86 ones do (see https://github.com/actions/runner-images/blob/main/images/windows/Windows2025-Readme.md)
     - name: Install cargo (Windows-arm)
       run: choco install rust
       if: matrix.os == 'windows-11-arm'