[backport 2.91] fix: macOS build broken by fatal lowdown CLI sandbox setup

This failed due to https://github.com/NixOS/nixpkgs/pull/346945, which
makes a second lowdown-unsandboxed that works in nix builds, and the
regular lowdown has executables that fail closed when the sandbox setup
fails.

The actual failure here is only visible on nixos-unstable at the moment,
not 24.05, but this commit should fix it up for all versions.

Fixes: #547
Change-Id: I50c0ecb59518ef01a7c0181114c1b4c5a7c6b78b
(cherry picked from commit a020f5f6cb)

package.nix

@@ -30,6 +30,8 @@
   lix-clang-tidy ? null,
   llvmPackages,
   lsof,
+  # FIXME: remove default after dropping NixOS 24.05
+  lowdown-unsandboxed ? lowdown,
   lowdown,
   mdbook,
   mdbook-linkcheck,
@@ -221,7 +223,7 @@ stdenv.mkDerivation (finalAttrs: {
       cmake
     ]
     ++ [
-      (lib.getBin lowdown)
+      (lib.getBin lowdown-unsandboxed)
       mdbook
       mdbook-linkcheck
     ]