Add trailing commas on addFlag incantations (backport #12967)

[mergify]

cc @Ericson2314, from #12957

Motivation

Context

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

---

This is an automatic backport of pull request #12967 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 06acbd3 has failed:

On branch mergify/bp/2.25-maintenance/pr-12967
Your branch is up to date with 'origin/2.25-maintenance'.

You are currently cherry-picking commit 06acbd37b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   src/libcmd/command.cc
	modified:   src/libcmd/common-eval-args.cc
	modified:   src/libmain/common-args.cc
	modified:   src/libmain/shared.cc
	modified:   src/libstore/globals.cc
	modified:   src/nix/build.cc
	modified:   src/nix/bundle.cc
	modified:   src/nix/derivation-show.cc
	modified:   src/nix/develop.cc
	modified:   src/nix/env.cc
	modified:   src/nix/prefetch.cc
	modified:   src/nix/sigs.cc
	modified:   src/nix/store-delete.cc
	modified:   src/nix/store-gc.cc
	modified:   src/nix/upgrade-nix.cc
	modified:   src/nix/verify.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libcmd/installables.cc
	both modified:   src/nix/copy.cc
	both modified:   src/nix/flake.cc

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally