shell.nix: conform to documentation

According to doc/manual/src/contributing/hacking.md, all development
shells should also be available in classic nix using the -A argument
to nix-shell, e.g.

  nix-shell -A native-clangStdenvPackages

This was not actually the case; flake-compat generates attrsets like

  nix-shell -A devShells.x86_64-linux.native-clangStdenvPackages

instead. These are unwieldy to use, so rather than changing the docs,
this changes the shell.nix file to result in the documented behavior.

Change-Id: I0920ccbdfddacf371f0aeaae7e290db65cf76ee7

shell.nix

@@ -1,3 +1,7 @@
-(import (fetchTarball "https://github.com/edolstra/flake-compat/archive/master.tar.gz") {
-  src = ./.;
-}).shellNix
+let
+  shell =
+    (import (fetchTarball "https://github.com/edolstra/flake-compat/archive/master.tar.gz") {
+      src = ./.;
+    }).shellNix;
+in
+shell.default // shell.devShells.${builtins.currentSystem}