deps: Add coreos-installer
I plan to migrate the installer test to mantle and it will
use this.  And eh let's include it on general principle so
people using cosa have it automatically.
@@ -67,3 +67,8 @@ strace
# Used to validate the meta.json schema
python3-jsonschema
+
+# Will be used by mantle, and eh let's include it on
+# general principle so people using cosa have it
+# automatically
+coreos-installer
