package: don't run shellHook in nested nix-shells

If a nested nix-shell is run inside a nix-shell, then the outer shell's
shellHook will be passed through and run again, unless the nested shell
defines its own.

With lix's hook, this can be annoying: forgetting to exit its nix-shell,
cd'ing to another repository & entering a nested nix-shell will happily
install lix's pre-commit hook in it.

This change makes lix's hook return early in such cases.

Change-Id: I91cb6eb6668f3a8eace36ecbdb01eb367861d77b

package.nix

@@ -414,6 +414,8 @@ stdenv.mkDerivation (finalAttrs: {
         glibcFix
         // {
 
+          name = "lix-shell-env";
+
           inputsFrom = [ finalAttrs ];
 
           # For Meson to find Boost.
@@ -437,6 +439,11 @@ stdenv.mkDerivation (finalAttrs: {
             ++ finalAttrs.checkInputs;
 
           shellHook = ''
+            # don't re-run the hook in (other) nested nix-shells
+            if [[ $name != lix-shell-env ]]; then
+              return;
+            fi
+
             PATH=$prefix/bin:$PATH
             unset PYTHONPATH
             export MANPATH=$out/share/man:$MANPATH