nixos/cron: Fix ShellCheck issue in preStart script

Fixes the following warning when setting
`systemd.enableStrictShellChecks = true`:

> SC2174 (warning): When used with -p, -m only applies to the deepest
> directory.

Since it could conceivably be possible for `/var` to not exist when
running this script, the fix also explicitly tries to create /var with
the normal 0755 permissions.

nixos/modules/services/scheduling/cron.nix

@@ -125,7 +125,8 @@ in
         wantedBy = [ "multi-user.target" ];
 
         preStart = ''
-          mkdir -m 710 -p /var/cron
+          (umask 022 && mkdir -p /var)
+          (umask 067 && mkdir -p /var/cron)
 
           # By default, allow all users to create a crontab.  This
           # is denoted by the existence of an empty cron.deny file.