config.boot.loader: Add timestampFormat

[non-bin]

Adds the option config.boot.loadertimestampFormat to change how the build timestamp is displayed in bootloaders, as discussed here https://www.reddit.com/r/NixOS/comments/17ilg97/anyone_know_how_to_show_the_time_of_configuration/

This is my first pull request so I'm not sure if it's alright that I skipped some things, like I don't think it needs test (I can't find any existing tests for the modules anyway), and so I need to update the changelog?
Thanks :)

Things done

• Add option to loader
• Inherit option into params of substituteAll in .nix files
• Add template strings to build scripts

• Build settings tested

  • All on UEFI x86_64-linux VM
  • Grub
    • timestampFormat = "%F %H:%M";
    • timestampFormat unset;
  • systemd-boot
    • timestampFormat = "%F %H:%M";
    • timestampFormat unset;

• 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/)

• 25.05 Release Notes (or backporting 24.11 and 25.05 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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[Flameopathic]

Not entirely sure about commit conventions here, given that changes are being made in three modules, but my best guess is that your commit would best be titled nixos/loader: Add timestampFormat option

[non-bin]

Thanks for the tip, I didn't really understand what it meant but I think I do now, thanks