gh-actions-cache: run tests

The tests don't need network, but a dummy GH_TOKEN.

pkgs/tools/misc/gh-actions-cache/default.nix

···
     "-w"
   ];
 
-  # Tests need network
-  doCheck = false;
+  # Needed for tests.
+  # https://github.com/actions/gh-actions-cache/issues/53#issuecomment-1464954495
+  env.GH_TOKEN = "dummy-token-to-facilitate-rest-client";
 
   meta = {
     description = "gh extension to manage GitHub Actions caches";