Disable benchmarks on forks (lit#1341)

Unfortunately we can't currently run benchmarks on PRs from forked repos, because the tachometer action reports results by posting a comment, which requires a github token that isn't available.

.github/workflows/benchmarks.yml

@@ -5,6 +5,12 @@ on: [pull_request]
 jobs:
   benchmarks:
     name: benchmarks
+
+    # We can't currently run benchmarks on PRs from forked repos, because the
+    # tachometer action reports results by posting a comment, and we can't post
+    # comments without a github token.
+    if: github.event.pull_request == null || github.event.pull_request.head.repo.full_name == github.repository
+
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2