mkimage-arch: Kill gpg-agent started in the chroot env

Add `pkill gpg-agent || true` after pacman-key command in chroot env
to kill gpg-agent if it's running.

Arch Linux's gnuepg package has just been updated to 2.1, which does
auto-start gpg-agent as documented(*1). pacman-key command depends
on gnupg and running it in the chroot environment causes gpg-agent
to start. This will make chroot to fail to umount $ROOTFS/dev because
gpg-agent seems to open it.

*1: https://gnupg.org/faq/whats-new-in-2.1.html#autostart

Signed-off-by: Tatsuya Kawano <tatsuya@hibaridb.org>
Merge branch 'mkimage-arch-gpg-agent' of github.com:tatsuya6502/docker into mkimage-arch-gpg-agent