nixos/alloy: set After=network.target

[flokli]

This prevents the network from being torn down before alloy itself it stopped.

Alloy tries to flush its WAL on SIGTERM, and if the network stack is already down, it tries to flush logs up to 90s during shutdown/reboot.

From https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/:

network.target has very little meaning during start-up. It only
indicates that the network management stack is up after it has been
reached. Whether any network interfaces are already configured when
it is reached is undefined. Its primary purpose is for ordering
things properly at shutdown: since the shutdown ordering of units in
systemd is the reverse of the startup ordering, any unit that is order
After=network.target can be sure that it is stopped before the network
is shut down if the system is powered off. This allows services to
cleanly terminate connections before going down, instead of abruptly
losing connectivity for ongoing connections, leaving them in an
undefined state. […]

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/)
• 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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