heaptrack: enable rustc-demangle

Rust is switching to their own symbol mangling convention (V0) which
requires dynamically loading the rustc-demangle library in heaptrack.

https://blog.rust-lang.org/2025/11/20/switching-to-v0-mangling-on-nightly/

pkgs/by-name/he/heaptrack/package.nix

@@ -11,6 +11,7 @@
   sparsehash,
   zstd,
   kdePackages,
+  rustc-demangle,
 }:
 
 stdenv.mkDerivation {
@@ -38,6 +39,7 @@ stdenv.mkDerivation {
     libunwind
     sparsehash
     zstd
+    rustc-demangle
   ]
   ++ (with kdePackages; [
     qtbase
@@ -53,6 +55,11 @@ stdenv.mkDerivation {
     elfutils
   ];
 
+  postPatch = ''
+    substituteInPlace src/interpret/demangler.cpp \
+      --replace-fail "librustc_demangle.so" "${rustc-demangle}/lib/librustc_demangle.so"
+  '';
+
   postInstall = lib.optionalString stdenv.hostPlatform.isDarwin ''
     makeWrapper \
       $out/Applications/KDE/heaptrack_gui.app/Contents/MacOS/heaptrack_gui \