ci: use store path hash for deploy spec artifact name

[dixslyf]

The status quo uses the commit hash ${{ github.sha }} in the deploy spec artifact name to differentiate different CI runs. However, this causes unnecessary builds when a new commit does not modify the NixOS hosts in any way. For example, modifying the latex-apa7 template does not affect any of the NixOS hosts, but because the commit hash is used, they get rebuilt anyway (not exactly rebuilt, since they should already be cached, but there is still 10–15 minutes wasted fetching from the cache to verify that the build succeeds).

If, instead, the same hash in the deploy spec's store path is used in the artifact name, many redundant builds can be skipped. [skip ci] would not need to be written in merge commit message bodies either.