README: the nightly's badge

Passing or failing for the last scheduled run on quattro. Runs started
by hand, on a branch or not, do not change it.

README.md

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 # Omarchy ISO
 
+[![Nightly ISO Build](https://github.com/omacom/omarchy-iso/actions/workflows/nightly-build.yml/badge.svg?branch=quattro&event=schedule)](https://github.com/omacom/omarchy-iso/actions/workflows/nightly-build.yml?query=branch%3Aquattro+event%3Aschedule)
+
 The Omarchy ISO is the only supported way to install Omarchy. It ships the Omarchy Configurator, installs Arch Linux, installs the Omarchy packages from the bundled mirror, runs target system setup in the chroot, creates the user, and runs `omarchy-setup-user` for that user.
 
 ## Downloading the latest ISO