modules/sops: allow forcing systemd-based activation

[lschuermann]

By allowing users to optionally force using systemd unit-based activation, they can inject dependencies on other services or mountpoints (for instance, when the age key is not stored on the root file system).

[lschuermann]

@Mic92 Only after opening this I realized this is effectively a duplicate of #39. Can either this or the other PR be merged? I have my key files stored on a persistent partition while the rest of the system is wiped each reboot, and with the activation-script based method sops-nix apparently always runs before that file system is mounted...

[Mic92]

Reverted because it broke evaluation: #857

[lschuermann]

Reverted because it broke evaluation: #857

Huh? I'm confused by this error. I've run nix flake check and have been using this revision on my own systems without issues. Also, the default for the introduced option is a verbatim copy of the let ... in expression before my change. So I'm puzzled as to how this could be breaking for anyone. From the error it looks like config.system is not defined?

If either you @Mic92 or @l0b0 can tell me how to produce this issue, I can try to come up with a different solution to this issue...