mono: set meta.mainProgram

[UlyssesZh]

Very simple PR for setting mono.meta.mainProgram = "mono";.

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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[nixpkgs-ci]

Backport failed for release-24.11, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-24.11
git worktree add -d .worktree/backport-358022-to-release-24.11 origin/release-24.11
cd .worktree/backport-358022-to-release-24.11
git switch --create backport-358022-to-release-24.11
git cherry-pick -x 17a8a6b7e475d78c3eea4f9112759a8649088a08 4d2a2ff20dd9c9ac96694261a87ed44fb11a50ea

[vcunat]

Oh, I didn't notice that there are two commits in this PR. Anyway, backport in #369229