* Introduce shell.nix to expose the Nix tooling, if required.
* dev-env: Rewrite the ghcide script to use nix-shell.
* language-support/hs: Use `nix-shell` for export-package.sh.
* Nix: Add a shebang to the Bazel wrapper script.
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* dev-env: Standardize the `set` header in `ghcide`.