fix: mount writable tmpfs /tmp in CI container

Docker --user runner means /tmp (created as root during build) isn't
writable. Bun requires a writable tempdir for any operation including
compilation. Mount a fresh tmpfs at /tmp with exec permissions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/evals.yml

@@ -62,7 +62,7 @@ jobs:
       credentials:
         username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
-      options: --user runner
+      options: --user runner --tmpfs /tmp:exec
     timeout-minutes: 20
     strategy:
       fail-fast: false