ci: only run update-kani-metrics in model-checking/verify-rust-std

Fork repos will auto run this job, thus bot PRs are weekly created
due to autoharness and frequent API changes in libstd.

So this commit disables the job for fork repos.

Author: zjp-CN

.github/workflows/kani-metrics.yml

@@ -11,6 +11,7 @@ defaults:
 
 jobs:
   update-kani-metrics:
+    if: github.repository == 'model-checking/verify-rust-std'
     runs-on: ubuntu-latest
 
     steps: