dev-shell: Disable separateDebugInfo

This breaks gdb pretty-printers inserted into .debug_gdb_scripts section,
because it implies --compress-debug-sections=zlib, -Wa,--compress-debug-sections.
This is very unfortunate, because then gdb can't use pretty printers for
Boost.Unordered (which are very useful, since boost::unoredred_flat_map is
impossible to debug). This seems perfectly fine to disable in the dev-shell for
the time being.

See [1-3] for further references.

With this change I'm able to use boost's pretty-printers out-of-the box:

```
p *importResolutionCache
$2 = boost::concurrent_flat_map with 1 elements = {[{accessor = {p = std::shared_ptr<nix::SourceAccessor> (use count 5, weak count 1) = {
        get() = 0x555555d830a8}}, path = {static root = {static root = <same as static member of an already seen type>, path = "/"},
      path = "/derivation-internal.nix"}}] = {accessor = {p = std::shared_ptr<nix::SourceAccessor> (use count 5, weak count 1) = {
        get() = 0x555555d830a8}}, path = {static root = {static root = <same as static member of an already seen type>, path = "/"},
      path = "/derivation-internal.nix"}}}
```

When combined with a simple `add-auto-load-safe-path ~/code` in .gdbinit

[1]: https://gerrit.lix.systems/c/lix/+/3880
[2]: https://git.lix.systems/lix-project/lix/issues/1003
[3]: https://sourceware.org/pipermail/gdb-patches/2025-October/221398.html

packaging/dev-shell.nix

@@ -70,6 +70,9 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
 
     # We use this shell with the local checkout, not unpackPhase.
     src = null;
+    # Workaround https://sourceware.org/pipermail/gdb-patches/2025-October/221398.html
+    # Remove when gdb fix is rolled out everywhere.
+    separateDebugInfo = false;
 
     env = {
       # For `make format`, to work without installing pre-commit
@@ -93,6 +96,8 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
       ++ map (transformFlag "libcmd") (ignoreCrossFile pkgs.nixComponents2.nix-cmd.mesonFlags);
 
     nativeBuildInputs =
+      let
+        inputs =
           attrs.nativeBuildInputs or [ ]
           ++ pkgs.nixComponents2.nix-util.nativeBuildInputs
           ++ pkgs.nixComponents2.nix-store.nativeBuildInputs
@@ -125,6 +130,10 @@ pkgs.nixComponents2.nix-util.overrideAttrs (
             lib.hiPrio pkgs.buildPackages.clang-tools
           )
           ++ lib.optional stdenv.hostPlatform.isLinux pkgs.buildPackages.mold-wrapped;
+      in
+      # FIXME: separateDebugInfo = false doesn't actually prevent -Wa,--compress-debug-sections
+      # from making its way into NIX_CFLAGS_COMPILE.
+      lib.filter (p: !lib.hasInfix "separate-debug-info" p) inputs;
 
     buildInputs = [
       pkgs.gbenchmark