This change avoids depending on /bin/sh in a test and thus makes it possible to use the test driver in a system that doesn't have /bin/sh. Now, the module pulls in bash directly instead of delegating figuring out which shell to use to env.

This enables me to write a test that ensures there is no bash in the closure besides the one from the test driver.

Ran about 20 systemd tests so I'm very confident that this works just as well as before.

Part #428908

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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