varnish: use jemalloc instead of glibc's malloc on linux.

this is the recommanded default, as glibc cause memory leaks:

"We ran into a problem with glibc's malloc on Linux where it seemed
like it failed to ever give memory back to the OS, causing the system
to swap. We have now switched to jemalloc which appears not to have
this problem." (from varnish-cache/doc/changes.rst)

pkgs/servers/varnish/default.nix

···
-{ lib, stdenv, fetchurl, fetchpatch, pcre, pcre2, libxslt, groff, ncurses, pkg-config, readline, libedit, coreutils
-, python3, makeWrapper }:
+{ lib, stdenv, fetchurl, fetchpatch, pcre, pcre2, jemalloc, libxslt, groff, ncurses, pkg-config, readline, libedit
+, coreutils, python3, makeWrapper }:
 
 let
   common = { version, sha256, extraNativeBuildInputs ? [] }:
···
         libxslt groff ncurses readline libedit makeWrapper python3
       ]
       ++ lib.optional (lib.versionOlder version "7") pcre
-      ++ lib.optional (lib.versionAtLeast version "7") pcre2;
+      ++ lib.optional (lib.versionAtLeast version "7") pcre2
+      ++ lib.optional stdenv.hostPlatform.isLinux jemalloc;
 
       buildFlags = [ "localstatedir=/var/spool" ];