Give unit tests access to a $HOME directory

Also, don't try to access cache.nixos.org in the libstore unit tests.

src/libstore-tests/meson.build

@@ -100,6 +100,7 @@ test(
   this_exe,
   env : {
     '_NIX_TEST_UNIT_DATA': meson.current_source_dir() / 'data',
+    'HOME': meson.current_build_dir() / 'test-home',
   },
   protocol : 'gtest',
 )