TESTING: cache in linux-old

I suspect the dependencies aren't available in the image, but we'll find
out shortly.

.github/workflows/linux-old.yml

@@ -58,6 +58,11 @@ jobs:
     container: 'debian:stretch'
 
     steps:
+      - uses: actions/cache@v4
+        id: cache
+        with:
+          path: ~/packagecache
+
       - name: 'install prereqs'
         # Remember, this shell is dash, not bash
         run: |