Revert GitHub actions back to macos-latest (#1377)

According to cachix/install-nix-action#183 the
MacOS problem should be resolved by #1376

.github/workflows/continuous-integration.yml

@@ -17,14 +17,13 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-          # MacOS 13 because of https://github.com/cachix/install-nix-action/issues/183
-          - macos-13
+          - macos-latest
         rust_channel:
           - stable
         include:
           - os: ubuntu-latest
             system: x86_64-linux
-          - os: macos-13
+          - os: macos-latest
             system: x86_64-darwin
             continue-on-error: true