net-firewall/nftables: Touch rules-save with right umask

net-firewall/nftables: Touch rules-save with right umask

The nftables ebuild contains code to ensure the rules-save
file is created so the service will start on systemd based
systems.

The current code creates the file with default permissions
644 which triggers the code for detecting misconfigured system
added to address bug #691326

Instead of just using touch, start a subshell so we can call
umask beforehand and address the issue.

Bug: https://bugs.gentoo.org/691326
Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>
Package-Manager: Portage-2.3.76, Repoman-2.3.11
Signed-off-by: Matthew Thode <prometheanfire@gentoo.org>