nbstripout: fix build on darwin

pkgs/applications/version-management/nbstripout/default.nix

@@ -1,4 +1,4 @@
-{lib, python2Packages, git, mercurial}:
+{lib, python2Packages, git, mercurial, coreutils}:
 
 with python2Packages;
 buildPythonApplication rec {
@@ -17,6 +17,12 @@ buildPythonApplication rec {
     sha256 = "126xhjma4a0k7gq58hbqglhb3rai0a576azz7g8gmqjr3kl0264v";
   };
 
+  # for some reason, darwin uses /bin/sh echo native instead of echo binary, so
+  # force using the echo binary
+  postPatch = ''
+    substituteInPlace tests/test-git.t --replace "echo" "${coreutils}/bin/echo"
+  '';
+
   # ignore flake8 tests for the nix wrapped setup.py
   checkPhase = ''
     PATH=$PATH:$out/bin:${mercurial}/bin pytest --ignore=nix_run_setup.py .