CI: add OBS workflow integration to build packages on push to main

When main is updated, packages for Arch/Debian/Ubuntu/SUSE will be built and published
on OBS:

https://download.opensuse.org/repositories/home:/bluca:/systemd/
https://build.opensuse.org/project/show/home:bluca:systemd

.obs/workflows.yml

@@ -0,0 +1,10 @@
+rebuild:
+  steps:
+    - trigger_services:
+        project: home:bluca:systemd
+        package: systemd
+  filters:
+    event: push
+    branches:
+      only:
+        - main