Skip renovate updating bufbuild/ actions

In #446, renovate is pinning our bufbuild/ actions, which doesn't seem
necessary.

Author: stefanvanburen

.github/renovate.json5

@@ -15,6 +15,16 @@
       ],
       "groupName": "GitHub Actions"
     },
+    {
+      "matchManagers": [
+        "github-actions"
+      ],
+      "matchPackagePrefixes": [
+        "bufbuild/"
+      ],
+      // We manage bufbuild actions ourselves; no need to pin or bump.
+      "enabled": false
+    },
     {
       "matchManagers": [
         "pep621"