get-stack.sh: `arch` command not found

[conao3]

General summary/comments (optional)

arch command is not installed at default, we should use uname -m more portable.

Steps to reproduce

$ curl -sSL https://get.haskellstack.org/ | sh
Detected Linux distribution: manjarolinux

This installer doesn't support your Linux distribution, trying generic
bindist...

main: line 74: arch: command not found
main: line 76: arch: command not found

related issues

google/fscrypt#92
asdf-community/asdf-haskell#21