github/workflows: Free up more disk space on zephyr workflow.

Zephyr CI is currently failing, because the runner is out of space.

Free up some more space using the existing plugin.

Fixes issue #18290.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/ports_zephyr.yml

@@ -28,11 +28,14 @@ jobs:
     - uses: jlumbroso/free-disk-space@main
       with:
         # Only free up a few things so this step runs quickly.
+        # (android would save 9.6GiB, but takes about 13m)
+        # (large-packages would save 4.6GiB, but takes about 3m)
         android: false
         dotnet: true
         haskell: true
         large-packages: false
         docker-images: false
+        tool-cache: true
         swap-storage: false
     - uses: actions/checkout@v5
     - id: versions