nixos/borgbackup: Add a persistent option

[blargg]

Persistent starts the backup service on power on if it was missed while
the system was powered down, for example.

Motivation for this change

Adds an option to make a backup job start on boot if it was missed during power off. This is useful for backups on personal machines where you might have the machine off during the normally scheduled backup.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS (n/a)
  • other Linux distributions (n/a)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/) (tested the service to make sure it runs)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[blargg]

This version follows along with how nix-gc allows for these parameters

nixpkgs/nixos/modules/services/misc/nix-gc.nix

Line 50 in 83d907f

persistent = mkOption {

.

[SuperSandro2000]

Please follow our commit naming guide: nixos/borgbackup: Add a persistent option.

[blargg]

Updated the formatting and the commit name.

[blargg]

Not sure if we want to act on the following as part of this PR, but will mention this here either way:
Since we add the ability to start on boot, it's a lot more likely that we run into the case where you try to back up to a remote server before the network starts up, causing the run to fail.

It seems like this is fixable by adding

wants = [ "network-online.target" ];
after = [ "network-online.target" ];

to either the timer or service. I think we could automatically detect remote usage by checking for "@" in the string for the repo, and possibly provide a setting to override.

[Stunkymonkey]

there is a previous PR #75056. I would prefer calling it persistentTimer, to point to the relationship with the timer unit (like @mweinelt suggested).

The rest looks fine to me.

[Stunkymonkey]

adding a description would be nice as well

[Stunkymonkey]

@mweinelt is setting the default value to true desirable?

[Stunkymonkey]

fixed in #153346