nixos/tmp: Fix format of /tmp mount options

[r-vdp]

Motivation for this change

The mount options need to be passed as a comma-separated list of options so that they end up one a single Options line in the resulting mount unit.
The current code passed the options as a list, resulting in several Options lines in the mount unit, all but the first of these were ignored by systemd however.
This behaviour is not clearly defined in the systemd man page.

This change might require a mention in the release notes as it will cause mount options to be applied where before they weren't.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.05 Release Notes (or backporting 21.11 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[vcunat]

LGTM.

Though I wonder if we should better address this (also) on a more general level by flattening the list even if anyone (including user config) specifies a list in similar NixOS options.

[ajs124]

This behaviour is not clearly defined in the systemd man page.

Has systemd upstream been made aware of this? IME they tend to fix documentation errors relatively fast.