python3Packages.pip: fix shell completions on cross (#567774)

pkgs/development/python-modules/pip/default.nix

@@ -2,11 +2,13 @@
   lib,
   buildPythonPackage,
   fetchFromGitHub,
-  pythonAtLeast,
 
   # build-system
   flit-core,
   installShellFiles,
+  stdenv,
+  buildPackages,
+  python,
 
   # docs
   sphinx,
@@ -104,14 +106,19 @@ let
       werkzeug
     ];
 
-    postInstall = ''
-      installManPage docs/build/man/*
-
-      installShellCompletion --cmd pip \
-        --bash <($out/bin/pip completion --bash --no-cache-dir) \
-        --fish <($out/bin/pip completion --fish --no-cache-dir) \
-        --zsh <($out/bin/pip completion --zsh --no-cache-dir)
-    '';
+    postInstall =
+      let
+        emulator = stdenv.hostPlatform.emulator buildPackages;
+      in
+      ''
+        installManPage docs/build/man/*
+      ''
+      + lib.optionalString (stdenv.hostPlatform.emulatorAvailable buildPackages) ''
+        installShellCompletion --cmd pip \
+          --bash <(${emulator} ${python.interpreter} $out/bin/pip completion --bash --no-cache-dir) \
+          --fish <(${emulator} ${python.interpreter} $out/bin/pip completion --fish --no-cache-dir) \
+          --zsh <(${emulator} ${python.interpreter} $out/bin/pip completion --zsh --no-cache-dir)
+      '';
 
     passthru.tests = {
       inherit pip-tools;