gdb: Look for debug info in /run/current-system/sw/lib/debug

The previous default was $out/lib/debug, which wasn't very useful.

This ensures that you can do

environment.systemPackages = [ pkgs.hello.debug ];

to install debug info.

nixos/modules/config/system-path.nix

···
       [ "/bin"
         "/etc/xdg"
         "/info"
-        "/lib"
+        "/lib" # FIXME: remove
+        #"/lib/debug/.build-id" # enables GDB to find separated debug info
         "/man"
         "/sbin"
         "/share/applications"

pkgs/development/tools/misc/gdb/default.nix

···
   configureFlags = with stdenv.lib;
     '' --with-gmp=${gmp} --with-mpfr=${mpfr} --with-system-readline
        --with-expat --with-libexpat-prefix=${expat}
+       --with-separate-debug-dir=/run/current-system/sw/lib/debug
     ''
     + optionalString (target != null) " --target=${target.config}"
     + optionalString (elem stdenv.system platforms.cygwin) "  --without-python";