Consume self-deploy's startAt attribute

[wpcarro]

As #157879 points-out, this attribute appears unused.

Fixes #157879

[tazjin]

commit message should be changed to something like nixos/self-deploy: .... bla ...

[lambdadog]

Good catch! Must have missed it when I rewrote the service for nixpkgs. I admit I'm happy to see actual use of the self-deploy service in the wild -- I've meant to write a blog post about it for ages but have never gotten around to it.

All looks good to me.

My personal preferences would prefer inherit (cfg) startAt; as I feel it makes it ever so slightly more transparent that the option is a direct passthrough to systemd.services.<name> for readers of the code but I'll be the first to admit that's just my own personal convention and I wouldn't be torn up if you prefer yours.

[lambdadog]

Looking back on the code, it may also be worthwhile to make the minor change of gating the service's systemd dependency on cfg.switchCommand being "boot" if only for correctness' purpose.