Don't purge the seed cache for PRs syncing clones

[katef]

As of #429, sync PRs to forks fail with a permissions error for purging the seed cache:

Fuzz
Purge cached seeds (mode m-RELEASE)
Run set +e
  set +e
  gh extension install actions/gh-actions-cache
  gh actions-cache delete seeds-m-RELEASE -R <org>/libfsm --confirm
  shell: /usr/bin/bash -e {0}
  env:
    pcre2: pcre2-10.40
    wc: wc
    seeds: seeds
    build: build
    cvtpcre: build/test/retest
    prefix: prefix
    maintainer: kate@elide.org
    GH_TOKEN: ***

Error: Resource not accessible by integration
Error: Process completed with exit code 1.

This PR is a simple fix that just skips using the cache for PRs where the repo isn't mine.

So fuzzing still runs, just with empty seeds. I think that's a good balance between serving as a smoke test and, well, working.