nixVersions: fix impurity trying to access the Nix daemon in libstore-tests

This failed on macOS:

nix-store-tests-run> C++ exception with description "../nix_api_store.cc:33: nix_err_code(ctx) != NIX_OK, message: error: getting status of '/nix/var/nix/daemon-socket/socket': Operation not permitted" thrown in the test body.

(cherry picked from commit 778156072444fdd06812b89e86169bf45f2425df)

pkgs/tools/package-management/nix/modular/src/libstore-tests/package.nix

···
           }
           (''
             export _NIX_TEST_UNIT_DATA=${data + "/src/libstore-tests/data"}
+            export NIX_REMOTE=$HOME/store
             ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage} \
               --gtest_filter=-${lib.concatStringsSep ":" finalAttrs.excludedTestPatterns}
             touch $out