MAINT: Update dependabot.yml file

Dependabot was failing and stopped running about 2 months ago. The
problem seems to be a failure to detect the semantic version of
`bus1/cabuild/`, so try to fix that by excluding that dependency.

[skip CI]

Author: charris

.github/dependabot.yml

@@ -8,3 +8,5 @@ updates:
       prefix: "MAINT"
     labels:
       - "03 - Maintenance"
+    ignore:
+      - dependency-name: "bus1/cabuild"