varnish: fix localstatedir for varnish* tools (#17508)

The varnish tools (varnishstat, varnishlog, ...) tried to load the VSM
file from a spurious var directory in the Nix store. Fix the default so
the tools "just work" when also keeping services.varnish.stateDir at the
default.

Notes:
- The tools use $localstatedir/$HOSTNAME so I've adapted the default for
stateDir as well to contain hostName.
- Added postStop action to remove the localstatedir. There is no point
in keeping it around when varnish does not run, as it regenerates it
on startup anyway.

Fixes #7495

nixos/modules/services/web-servers/varnish/default.nix

···
       };
 
       stateDir = mkOption {
-        default = "/var/spool/varnish";
+        default = "/var/spool/varnish/${config.networking.hostName}";
         description = "
           Directory holding all state for Varnish to run.
         ";
···
       preStart = ''
         mkdir -p ${cfg.stateDir}
         chown -R varnish:varnish ${cfg.stateDir}
+      '';
+      postStop = ''
+        rm -rf ${cfg.stateDir}
       '';
       path = [ pkgs.gcc ];
       serviceConfig.ExecStart = "${pkgs.varnish}/sbin/varnishd -a ${cfg.http_address} -f ${pkgs.writeText "default.vcl" cfg.config} -n ${cfg.stateDir} -u varnish";

pkgs/servers/varnish/default.nix

···
   buildInputs = [ pcre libxslt groff ncurses pkgconfig readline python
     pythonPackages.docutils];
 
+  buildFlags = "localstatedir=/var/spool";
+
   meta = {
     description = "Web application accelerator also known as a caching HTTP reverse proxy";
     homepage = "https://www.varnish-cache.org";