makeBinaryWrapper: add --set-from-file option

[llakala]

I've been working on a collection of wrappers, and want each wrapper to support both:

1. directly passing in a config value, with some pkgs.formats transformation (think home-manager .settings options
2. an impure solution, where you pass the program a homedir path, and have it read from the state of that file at runtime

Most programs support taking a config path, but some can only read from the state of an env var, or the flags they've been passed directly. To allow these programs to be handled impurely, the wrapper itself needs to read from the file and pass the contents to the program.

This PR adds the ability to do the first part of that. Calling wrapProgram $out/bin/foo --set-from-file FOO_ENV_VAR /home/user/example/file creates the relevant C code to read from /home/user/example/file at runtime, and set FOO_ENV_VAR to the contents of this file. I'm using this locally with one of my wrappers, and can confirm it functions.

Feel free to critique my C code. In a future PR, I'd like to add the equivalent feature for --add-flags, and see if I can bring this API to the normal makeWrapper as well.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[adisbladis]

Something adjacent I'd find useful is to be able to source the equivalent of systemd's EnvironmentFile directive: https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#EnvironmentFile=.
Not a blocker for this feature addition though.