Revert "workflows/eval: disable swap"

This reverts commit f2648b263b.

While the idea to never use swap was fine, in practice this meant that
when nix ran OOM, some other process was killed instead. This lead to
the job not being possible to be cancelled anymore and thus needing to
timeout, before subsequent jobs could be scheduled. This can take up to
6 hours for GitHub Actions by default.

Re-enabling the swap file to catch this case more gracefully. It's still
the goal to never actually *use* the swap file during Eval and just a
safeguard.

Keeping the changed chunkSize and not reverting it - this makes it
slightly less likely to hit the swap file when running with Lix.

(cherry picked from commit 9cde368b4c)

.github/workflows/eval.yml

@@ -32,11 +32,16 @@ jobs:
     outputs:
       targetRunId: ${{ steps.targetRunId.outputs.targetRunId }}
     steps:
-      # We'd rather fail than run slow eval on swap.
-      # Failing allows us to adjust the chunkSize to remain fast.
-      - name: Disable swap
+      # This is not supposed to be used and just acts as a fallback.
+      # Without swap, when Eval runs OOM, it will fail badly with a
+      # job that is sometimes not interruptible anymore.
+      # If Eval starts swapping, decrease chunkSize to keep it fast.
+      - name: Enable swap
         run: |
-          sudo swapoff -a
+          sudo fallocate -l 10G /swap
+          sudo chmod 600 /swap
+          sudo mkswap /swap
+          sudo swapon /swap
 
       - name: Check out the PR at the test merge commit
         uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2