add cache-proxy test (#557)

test to verify that a cache stored in one repo can't be restored in another

Reviewed-on: https://code.forgejo.org/forgejo/end-to-end/pulls/557
Reviewed-by: Gusted <gusted@noreply.code.forgejo.org>
Reviewed-by: earl-warren <earl-warren@noreply.code.forgejo.org>
Co-authored-by: Kwonunn <kwonunnx@gmail.com>
Co-committed-by: Kwonunn <kwonunnx@gmail.com>

actions/example-cache-proxy/storer/.forgejo/workflows/store.yml

@@ -0,0 +1,17 @@
+on: [push]
+
+jobs:
+  build:
+    runs-on: docker
+    container:
+      image: code.forgejo.org/oci/node:20-bookworm
+    steps:
+      - name: create something
+        run: echo SOMETHING > /usr/local/bin/something
+
+      - name: cache save
+        uses: https://code.forgejo.org/actions/cache/save@v4
+        with:
+          path: |
+            /usr/local/bin/something
+          key: thecoolsecretcachekey