bash: Fix shellcheck lint

[l0b0]

Motivation for this change

Existing issues requesting ShellCheck compliance: #133088, #21166.

This change mostly prepares for ShellCheck by indicating which shell (Bash in all the cases I could find) ShellCheck should interpret the code as, and by configuring pre-commit to use the latest version of ShellCheck.

Let me know if this should be split up in some other way.

The EditorConfig errors seem to be from the existing source - I've create a separate PR (merged) to fix those.

Note to self: To list all paths with a specific issue, change .pre-commit-config.yaml to set e.g.

args: [--format=json, --include=SC1008]
require_serial: true

Then pre-commit run --all-files | tail +5 | jq --raw-output '.[] | .file' | xargs --max-args=1 idea-ultimate to pass all the files to IDEA.

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 wip"
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 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
• Fits CONTRIBUTING.md.

[l0b0]

@Artturin