libstore-tests: Fix impurity trying to access the Nix daemon

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.

src/libstore-tests/meson.build

@@ -101,6 +101,7 @@ test(
   env : {
     '_NIX_TEST_UNIT_DATA': meson.current_source_dir() / 'data',
     'HOME': meson.current_build_dir() / 'test-home',
+    'NIX_REMOTE': meson.current_build_dir() / 'test-home' / 'store',
   },
   protocol : 'gtest',
 )

src/libstore-tests/package.nix

@@ -77,6 +77,7 @@ mkMesonExecutable (finalAttrs: {
           }
           (''
             export _NIX_TEST_UNIT_DATA=${data + "/src/libstore-tests/data"}
+            export NIX_REMOTE=$HOME/store
             ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
             touch $out
           '');