fix: remove --tmpfs mount, rely on Dockerfile USER + chmod 1777 /tmp

The --tmpfs /tmp:exec mount replaces /tmp with a root-owned tmpfs,
undoing the chmod 1777 from the Dockerfile. Remove the tmpfs mount
so the Dockerfile's /tmp permissions persist at runtime.

Dockerfile already has USER runner and chmod 1777 /tmp, which should
give bun write access without any runtime workarounds.

Also removes the Fix temp dirs step since it's no longer needed.

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

.github/workflows/evals.yml

@@ -62,7 +62,6 @@ jobs:
       credentials:
         username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
-      options: --tmpfs /tmp:exec
     timeout-minutes: 20
     strategy:
       fail-fast: false
@@ -98,14 +97,6 @@ jobs:
         with:
           fetch-depth: 0
 
-      # Bun needs a writable tmpdir — set to /tmp which is a tmpfs mount
-      - name: Fix temp dirs
-        run: |
-          echo "TMPDIR=/tmp" >> "$GITHUB_ENV"
-          echo "BUN_TMPDIR=/tmp" >> "$GITHUB_ENV"
-          echo "XDG_CACHE_HOME=/tmp/.cache" >> "$GITHUB_ENV"
-          mkdir -p /tmp/.cache
-
       # Restore pre-installed node_modules from Docker image via symlink (~0s vs ~15s install)
       # If package.json changed since image was built, fall back to fresh install
       # Note: GITHUB_ENV vars from previous step are now active