Use SSH deploy key instead of PAT in nightly.yaml

[criccomini]

Describe the bug
The nightly.yaml currently uses my personal access token (that I've scoped to just the slatedb/slatedb-website repo) to commit changes to the slatedb-website repo. I'd rather not have to manage a PAT for this, but the benchmark-action/github-action-benchmark action doesn't currently support ssh-key.

Expected behavior
I'd rather use SSH deploy keys for nightly.yaml. To do so, this patch needs to get merged:

benchmark-action/github-action-benchmark#283