files: avoid surprises when linking files

[ncfavier]

In the unlikely (but possible) event that a directory is created in place of a target file after checkLinkTargets but before linkGeneration, currently the link is created inside the directory, which is surprising (see https://github.com/nix-community/home-manager/pull/3017 for context).

To avoid this, use ln -T and bail if it fails.

Also bail if backing up the target fails.

Ideally, checkLinkTargets and linkGeneration should be more tightly coupled.

Checklist

• [X] Change is backwards compatible.

• [X] Code formatted with ./format.

• [X] Code tested through nix-shell --pure tests -A run.all.

• [ ] Test cases updated/added. See example.

• [X] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

[ncfavier]

Do you know whether the GNU coreutil package is in the activation script's $PATH? If so, my previous comment can be ignored but a note should be left.

Yep. I'm not sure we want to add a comment in the code every time we rely on this though, it seems like a fine "ambient assumption".

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[ncfavier]

cc @rycee, I still think this change is good

[ncfavier]

I've removed the other change, added a comment and rebased on master.

Also note that we're already using ln -T: https://github.com/nix-community/home-manager/blob/c1a830c8fabb13f95f51ecf48552f0a794d8718a/modules/files.nix#L274