Test nix develop on fixed-output derivations

It half works today, we should fix this but also not regress it!

tests/functional/nix-shell.sh

@@ -279,3 +279,9 @@ assert (!(args ? inNixShell));
 (import $shellDotNix { }).shellDrv
 EOF
 nix-shell "$TEST_ROOT"/shell-ellipsis.nix --run "true"
+
+if [[ -z ${NIX_TESTS_CA_BY_DEFAULT:-} ]]; then
+  # `nix develop` should also work with fixed-output derivations
+  # shellcheck disable=SC2016
+  nix develop -f "$shellDotNix" fixed -c bash -c '[[ -n $stdenv ]]'
+fi

tests/functional/shell.nix

@@ -84,6 +84,16 @@ let
       '';
     };
 
+    # Shells should also work with fixed-output derivations
+    fixed = mkDerivation {
+      name = "fixed";
+      FOO = "was a fixed-output derivation";
+      outputHash = "1ixr6yd3297ciyp9im522dfxpqbkhcw0pylkb2aab915278fqaik";
+      outputHashMode = "recursive";
+      outputHashAlgo = "sha256";
+      outputs = [ "out" ];
+    };
+
     # Used by nix-shell -p
     runCommand =
       name: args: buildCommand: