Add Chainguard ecosystem. (#2288)

Co-authored-by: Andrew Pollock <[email protected]>

Author: oliverchang

osv/ecosystems/_ecosystems.py

@@ -55,6 +55,7 @@
     # handled separately in get().
     'AlmaLinux': OrderingUnsupportedEcosystem(),
     'Alpine': OrderingUnsupportedEcosystem(),
+    'Chainguard': OrderingUnsupportedEcosystem(),
     'Debian': OrderingUnsupportedEcosystem(),
     'Photon OS': OrderingUnsupportedEcosystem(),
     'Rocky Linux': OrderingUnsupportedEcosystem(),