The Arch CI is broken because it does not already have the package database downloaded like a regular system would. pacman -S and pacman -Syu (this one is debatable. who would want to upgrade during an installation script?) are safe, but pacman -Sy is not.

@noncombatant we need to update the CI config to special-case archlinux like we do here with others

I've just checked, and the installer script will fail on a fresh archlinux:latest image, with the error message advising to run pacman -Sy:

# bash install.sh
Installing Tailscale for arch, using method pacman
+ pacman -S tailscale --noconfirm
warning: database file for 'core' does not exist (use '-Sy' to download)
warning: database file for 'extra' does not exist (use '-Sy' to download)
error: target not found: tailscale

If it's not always safe to run that command, I think we have two options:

1. Only run the command when we need to (e.g. when database files are missing), assuming there is a stable way to determine that.
2. Completely remove the command from the installer script, and run it on CI before the script. This would not be my first choice, since ideally we'd want the script to do whatever is necessary (but not more) to install Tailscale.

We could do if !pacman -S tailscale ...; pacman -Sy ... (it does return with status 1 when it fails — for any reason, though, not just unknown packages), but I think that still might surprise people who don't want -Sy to happen without their intent. So @knyar 's option 2 might make CI most like what Arch users (and other pacman-users?) expect, which seems better.

1. Only run the command when we need to (e.g. when database files are missing), assuming there is a stable way to determine that.

On Arch and other rolling release pacman distributions, you'd still cause a partial upgrade. By using pacman -Sy, you promise that you will finish the upgrade.
Here's a better explanation of why you should never, ever, ever do this on an Arch system: https://bbs.archlinux.org/viewtopic.php?id=241092
The best thing to do would be to ensure the CI's database is up-to-date.