* install_kustomize: support linux/aarch64, with fallback to old behavior
* shellcheck
* Comments from first review
* Comments from review
* Review comments: message consistency
This is important to improve reliency of other automations/scripts that are using this script internally. Otherwise install script might stall forever in some cases.
The install script fails and thinks that alpine linux is in windows. This is because
`$OSTYPE` in alpine linux is linux-musl, not linux-gnu as this script assumes.
I tested these changes with this script:
```
set -euo pipefail
opsys=""
function check {
opsys=windows
if [[ "$OSTYPE" == linux* ]]; then
opsys=linux
elif [[ "$OSTYPE" == darwin* ]]; then
opsys=darwin
fi
}
OSTYPE="linux-gnu"
check
test "$opsys" == "linux" || echo $OSTYPE test failed
OSTYPE="linuxsomething"
check
test "$opsys" == "linux" || echo $OSTYPE test failed
OSTYPE="darwinsomething"
check
test "$opsys" == "darwin" || echo $OSTYPE test failed
OSTYPE="either"
check
test "$opsys" == "windows" || echo $OSTYPE test failed
```
ref: #2146
