feat(install.sh): add support for `--mainline` (default) and `--stable`#12858

mafredri merged 3 commits intomaincoder/coder:mainfrom

mafredri/feat-install.sh-add-stable-mainline-supportcoder/coder:mafredri/feat-install.sh-add-stable-mainline-supportCopy head branch name to clipboard