Onboard dependabot on Bazel (#616)

I hope this should work (looking at
https://docs.github.com/de/code-security/reference/supply-chain-security/dependabot-options-reference#package-ecosystem-).

Author: faximan

.github/dependabot.yml

@@ -9,3 +9,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+
+  - package-ecosystem: "bazel"
+    directory: "/"
+    schedule:
+      interval: "weekly"