Allow SSH deploy

Great action, thanks a lot for setting this up for everyone to use. I was wondering whether it is possible to use an SSH deploy key instead of a (more general) personal access token.

In particular, I'm using a separate repository for storing the benchmark results, but if I specify `gh-repository` the actions fails if I do not provide a token. It looks like using custom commits as described [here](https://github.com/benchmark-action/github-action-benchmark#charts-on-github-pages-1) would be the easiest way to achieve this, e.g. using [this action](https://github.com/marketplace/actions/push-directory-to-another-repository). Alternatively, and much cleaner, the action itself could make use of an SSH key provided for pushing to the remote repository.

Any thoughts?