tools: Drop cockpit-machines for Ubuntu 18.04

Ubuntu 18.04 does not have libvirt-dbus package so cockpit-machines
was using virsh to communicate with libvirt on that distro version.
We stopped adding new features to the cockpit-machines code implemented
with virsh long time now.
Let's just stop updating the package there.

tools/debian/rules

@@ -14,6 +14,12 @@ ifeq ($(BUILD_DOCKER),)
        export DH_OPTIONS = -Ncockpit-docker
 endif
 
+# Stop building cockpit-machines for ubuntu-1804 (backports);
+BUILD_MACHINES = $(findstring $(shell . /etc/os-release; echo $${VERSION_ID}),18.04)
+ifeq ($(BUILD_MACHINES),18.04)
+       export DH_OPTIONS = -Ncockpit-machines
+endif
+
 %:
 	dh $@