shell.nix: Support nix-shell -A

It used to be that `nix-shell -A hello` would launch the build shell
for the `hello` package.
By adding `/shell.nix`, that stopped working, as all versions of
`nix-shell` resolve the unspecified file to `$PWD/shell.nix` if it
exists, and now it does.

I have to admit that this use of `//` is not pretty, but the UX/DX
hard to match.

shell.nix

@@ -17,8 +17,17 @@
 }:
 let
   inherit (import ./ci { inherit nixpkgs system; }) pkgs;
+
+  # For `nix-shell -A hello`
+  curPkgs = builtins.removeAttrs (import ./. { inherit system; }) [
+    # Although this is what anyone may expect from a `_type = "pkgs"`,
+    # this file is intended to produce a shell in the first place,
+    # and a `_type` tag could confuse some code.
+    "_type"
+  ];
 in
-pkgs.mkShellNoCC {
+curPkgs
+// pkgs.mkShellNoCC {
   packages = with pkgs; [
     # The default formatter for Nix code
     # See https://github.com/NixOS/nixfmt