nixos/greenclip: restart daemon if it exits

[presto8]

The Linux out-of-memory killer sometimes kills greenclip, as seen in the system journal:

greenclip.service: A process of this unit has been killed by the OOM killer.

This changeset adjusts the OOM score to reduce the likelihood that the greenclip process will be killed. Additionally, the systemd restart policy is set to "always" for greenclip service if it does get killed.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[tomfitzhenry]

https://github.com/NixOS/nixpkgs/pull/303859/files#r1573673486