Be mindful about the default arguments passed to nixpkgs in nix-shell

Starting with 6eeb6f9 Nix started injecting `inNixShell`
unconditonally into all nix-shell calls. This caused to a minor breakage
with old Nix expressions that had a thin wrapper around nixpkgs. While
we can fix those projects we should try to be backwards compatible in
this regard.

The error encountered in those cases looked like this:
    error: anonymous function at /home/andi/dev/nixos/nix/tests/shell-empty-attrs-args.nix:1:1 called with unexpected argument 'inNixShell'
           at «string»:1:18:

                1| {...}@Args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (foo) (run) (echo "$(foo)") ]; } ""
                 |                  ^

           … while evaluating anonymous lambda

           at «string»:1:1:

                1| {...}@Args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (foo) (run) (echo "$(foo)") ]; } ""
                 | ^

           … from call site

src/nix-build/nix-build.cc

@@ -270,7 +270,12 @@ static void main_nix_build(int argc, char * * argv)
 
     if (packages) {
         std::ostringstream joined;
-        joined << "{...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) \"shell\" { buildInputs = [ ";
+        joined << "{...}@args: "
+               << "let nixpkgs = import <nixpkgs>; "
+               << "nixpkgsArgs = builtins.intersectAttrs (builtins.functionArgs nixpkgs) args; "
+               << "in "
+               << "with nixpkgs nixpkgsArgs; "
+               << "(pkgs.runCommandCC or pkgs.runCommand) \"shell\" { buildInputs = [ ";
         for (const auto & i : left)
             joined << '(' << i << ") ";
         joined << "]; } \"\"";

tests/nix-shell.sh

@@ -52,6 +52,11 @@ output=$(NIX_PATH=nixpkgs="$shellDotNix" nix-shell --pure -p foo bar --run 'echo
 output=$(NIX_PATH=nixpkgs="$shellDotNix" nix-shell --pure -p foo --argstr fooContents baz --run 'echo "$(foo)"')
 [ "$output" = "baz" ]
 
+# Test nix-shell -p with older versions of Nixpkgs that do not expect
+# the (new) isNixShell attribute.
+output=$(NIX_PATH=nixpkgs=shell-empty-attrs-args.nix nix-shell --pure -p foo --run 'echo "$(foo)"')
+[ "$output" = "zes" ]
+
 # Test nix-shell shebang mode
 sed -e "s|@ENV_PROG@|$(type -P env)|" shell.shebang.sh > $TEST_ROOT/shell.shebang.sh
 chmod a+rx $TEST_ROOT/shell.shebang.sh

tests/shell-empty-attrs-args.nix

@@ -0,0 +1 @@
+{ }: import ./shell.nix { fooContents = "zes"; }