v1.3 does not work with arm 64 architecture

[huan]

We have two reports from arm 64 users that the SMF can not start as expected:

• Use docker buildx to add arm platform to our docker image #76 (comment) from @mrsufgi
• Use docker buildx to add arm platform to our docker image #76 (comment) from @guysoft

Need to be investigated and fixed.

[gothchibjo]

Confirm for 1.4.7:
aarch64

[pd-giz-dave]

I have discovered three issues:

1. the install-s6-overlays.sh script always loads the amd version (its not using the $URL variable built in the previous line)
2. the -C option on the tar line is set to '\' (line continuation) and should be '/' (root)
3. version 2 of s6 only provides a binary for 'arm'

Modified install-s6-overlays.sh is below:
`#!/usr/bin/env bash

set -eo pipefail

if [[ -n "$1" ]]; then
S6_VERSION=$1; shift
else
echo "arg 1 should be the s6 version"
exit 1
fi

case $(uname -m) in
armv6l)
;& # fall through
armv7l)
ARCH=arm
;;
armv8l)
ARCH=arm
;;
aarch64)
ARCH=arm
;;
i386)
ARCH=x86
;;
x86_64)
ARCH=amd64
;;
*)
echo "ARCH not supported: $ARCH"
exit 1
;;
esac

URL="https://github.com/just-containers/s6-overlay/releases/download/v${S6_VERSION}/s6-overlay-${ARCH}.tar.gz"
curl -L -s "$URL" | tar xpzf - -C /
`

Running script/build.sh then creates a binary on my aarch64 system that works. Maybe update to version 3 of s6? (I tried but got tangled up in other problems).