ci: Always run with sandbox, even on Darwin

And fix a test failure in the sandbox due to /home
existing on Darwin but not being accessible in the sandbox since it's a
symlink to /System/Volumes/Data/home, see
https://github.com/NixOS/nix/actions/runs/4205378453/jobs/7297384658#step:6:2127:

    C++ exception with description "error: getting status of /home/schnitzel/darmstadt/pommes: Operation not permitted" thrown in the test body.

On Linux this wasn't a problem because there /home doesn't exist in the sandbox

.github/workflows/ci.yml

@@ -20,6 +20,9 @@ jobs:
       with:
         fetch-depth: 0
     - uses: cachix/install-nix-action@v20
+      with:
+        # The sandbox would otherwise be disabled by default on Darwin
+        extra_nix_config: "sandbox = true"
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
     - uses: cachix/cachix-action@v12
       if: needs.check_secrets.outputs.cachix == 'true'

src/libutil/tests/tests.cc

@@ -202,7 +202,7 @@ namespace nix {
     }
 
     TEST(pathExists, bogusPathDoesNotExist) {
-        ASSERT_FALSE(pathExists("/home/schnitzel/darmstadt/pommes"));
+        ASSERT_FALSE(pathExists("/schnitzel/darmstadt/pommes"));
     }
 
     /* ----------------------------------------------------------------------------