vivaldi: use coreutils from nixpkgs

On non-nixos systems the startup script uses coreutils commands linked
against a different glibc than exposed via our wrapped program. As a
result the following failure can occur when invoking `rm`, `readlink`,
`timeout`, and other coreutils commands:

```
+ timeout 3s /update-ffmpeg --user
timeout: symbol lookup error: /nix/store/pacbfvpzqz2mksby36awvbcn051zcji3-glibc-2.40-36/lib/libc.so.6: undefined symbol: __tunable_is_initialized, version GLIBC_PRIVATE
```

Repro:

1. Be on ubuntu 22.04[1]
2. `nix build --impure --expr 'with import ./. {config.allowUnfree=true;}; vivaldi.override { proprietaryCodecs = true; enableWidevine=true;}'`
3. `./result/bin/vivaldi`

[1]: I suspect other distros can exhibit the issue, but this is what I've tested on.

pkgs/applications/networking/browsers/vivaldi/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, zlib, libX11, libXext, libSM, libICE, libxkbcommon, libxshmfence
+{ lib, stdenv, coreutils, fetchurl, zlib, libX11, libXext, libSM, libICE, libxkbcommon, libxshmfence
 , libXfixes, libXt, libXi, libXcursor, libXScrnSaver, libXcomposite, libXdamage, libXtst, libXrandr
 , alsa-lib, dbus, cups, libexif, ffmpeg, systemd, libva, libGL
 , freetype, fontconfig, libXft, libXrender, libxcb, expat
@@ -114,6 +114,7 @@ in stdenv.mkDerivation rec {
       --set-default FONTCONFIG_FILE "${fontconfig.out}/etc/fonts/fonts.conf" \
       --set-default FONTCONFIG_PATH "${fontconfig.out}/etc/fonts" \
       --suffix XDG_DATA_DIRS : ${gtk3}/share/gsettings-schemas/${gtk3.name}/ \
+      --prefix PATH : ${coreutils}/bin \
       ${lib.optionalString enableWidevine "--suffix LD_LIBRARY_PATH : ${libPath}"}
   '' + lib.optionalString enableWidevine ''
     ln -sf ${widevine-cdm}/share/google/chrome/WidevineCdm $out/opt/${vivaldiName}/WidevineCdm