Make justfile commands available in the Nix development shell

[TyceHerrman]

Description

Add just to the default Nix development shell so contributors can use the repository's justfile without installing just separately. Keep GNU Make available as the existing fallback.

no flake.lock update required. just comes from the already-pinned nixpkgs input, and no flake input changes.

Closes #5881.

Testing

• nix-instantiate --parse flake.nix
• nix flake check
• nix develop --ignore-environment -c just --version
• nix develop --ignore-environment -c go version
• nix develop --ignore-environment -c just format
• nix develop --ignore-environment -c just build
• nix develop --ignore-environment -c just lint
• nix develop --ignore-environment -c just unit-test
• nix-shell --run 'just --version'

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide) — no test-code change needed; validated with the commands above
• Text is internationalised (see here) — no user-facing strings changed
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here) — no UserConfig entry added
• Docs have been updated if necessary — no documentation change needed
• You've read through your own file changes for silly mistakes etc

[stefanhaller]

Thanks; please never merge master into a branch, always rebase onto master instead. I did this for you this time. Merging.