systemd: make unit files with `runCommand`

[jsoo1]

Description

Fixes #3217

Avoids errors of the following sort when /tmp is mounted on a different drive.

mv: inter-device move failed: '/tmp/nix-build-emacs.service.drv-0/.attr-1' to "/nix/store/xyz-emacs.service'/emacs.service'"; unable to remove target: Permission denied

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

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

• [x] 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.

• If this PR adds a new module

  • [ ] Added myself as module maintainer. See example.

  • [ ] Added myself and the module files to .github/CODEOWNERS.

[jsoo1]

nix-shell --pure tests -A run.all failed for me??

Edit: Had to add -I nixpkgs=<path to nixpkgs>

Edit: Failed with error: attribute 'shellDryRun' missing, at .../modules/programs/bash.nix:12:9

Edit: switched to nixpkgs revision eb1f0837a05 and that seemed to work.

[jsoo1]

I believe this change is backwards-compatible but I am not 100% certain.

Edit: The compatibility would be modulo any changes to runCommand but I don't know the whole history of it.

[jsoo1]

Hm. I'm not 100% sure this actually fixes the issue. Simply bumping home-manager did the trick.

That said, I think this might be good to do?

[rycee]

Hmm, I think if this is an issue with writeTextFile then it needs to be fixed in Nixpkgs. Although, I'm not sure what could cause it. Seems like writeTextFile attempted to write the target file twice (the second time being after making Nix store read-only)? Quite curious.

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