Add which to all shell environments (#234)

GHC's build system (mk/boilerplate.mk:182) uses `which` to locate ghc.
After commit a354771 switched from recursive-nix `nix print-dev-env` to
`devShellTools.unstructuredDerivationInputEnv`, `which` was no longer
transitively included. This caused CI failures:

  make[1]: which: No such file or directory
  ../mk/boilerplate.mk:182: *** Cannot find ghc: .  Stop.

Add pkgs.which explicitly to nativeBuildInputs/buildInputs in all four
shell definitions (dynamic, static, cross-js, cross-windows).

Author: angerman

cross-js.nix

@@ -100,7 +100,9 @@ pkgs.mkShell ({
         (pkgs.pkg-config or pkgconfig)
         (tool "happy")
         (tool "alex")
-        stdenv.cc.cc.lib ]) ++ (with pkgs.buildPackages; [
+        stdenv.cc.cc.lib
+        which
+    ]) ++ (with pkgs.buildPackages; [
     ])
     ++ builtins.attrValues haskell-tools
     ++ pkgs.lib.optional withIOG

cross-windows.nix

@@ -205,7 +205,9 @@ pkgs.pkgsBuildBuild.mkShell ({
         (pkgsBuildBuild.pkg-config or pkgsBuildBuild.pkgconfig)
         (tool "happy")
         (tool "alex")
-        stdenv.cc.cc.lib ])
+        stdenv.cc.cc.lib
+        pkgsBuildBuild.which
+    ])
     ++ map pkgs.lib.getDev (with pkgs; [
         zlib pcre openssl
         windows.mcfgthreads

dynamic.nix

@@ -122,6 +122,7 @@ pkgs.mkShell {
         openssl
         pcre
         pkg-config
+        which
         zlib
       ])
       ++ optional stdenv.hostPlatform.isLinux pkgs.systemd

static.nix

@@ -158,7 +158,9 @@ pkgs.mkShell (rec {
         (compiler.override { enableShared = true; })
     ] ++ (with pkgs; [
         (pkgs.pkg-config or pkgconfig)
-        stdenv.cc.cc.lib ]) ++ (with pkgs.buildPackages; [
+        stdenv.cc.cc.lib
+        which
+    ]) ++ (with pkgs.buildPackages; [
     ])
     ++ builtins.attrValues haskell-tools
     ++ pkgs.lib.optional withIOG (with pkgs; [ cddl cbor-diag ])