I believe that since a remote store build doesn't download any built derivations to the original machine, the normal setup of e.g. the Cachix action won't work (whereas I think it would for normal remote builds?). So you would need to use the ability of nixbuild.net to upload to a cache.
Might also be good to have a section on "how to use this with cachix" in the nixbuild-action readme, since it's likely to be where people trying to set up GHA are looking. Could just link to other docs though.
I believe that since a remote store build doesn't download any built derivations to the original machine, the normal setup of e.g. the Cachix action won't work (whereas I think it would for normal remote builds?). So you would need to use the ability of nixbuild.net to upload to a cache.