Skip cache delete job for forked PRs

Dependabot and forked PRs don't have 'write' permission to delete the
GitHub Actions caches.

Skip this step for those kinds of PRs. It will keep the caches around
for longer than we'd like, but at least it won't balloon in size if we
skip this for all PRs.

[skip changeset]

Rakefile

@@ -197,6 +197,10 @@ namespace :build_matrix do
             unit_test_job_key,
             diagnose_job_key
           ],
+          # Skip this job for dependabot and forked PRs as they do not
+          # have write permissions to the repository to delete caches
+          "if" => "github.actor != 'dependabot[bot]' && " \
+            "github.event.pull_request.head.repo.fork == false",
           "runs-on" => "ubuntu-latest",
           "steps" => [
             {