ci: use get_charms_build_with_cache.yaml workflow

This workflow enables the usage of canonical/charmed-kubeflow-workflows/.github/workflows/get_charms_build_with_cache.yaml
for this CI, meaning that the integration tests and publish jobs are now able to download charm artefacts and use them
consistently throught the various operations of this CI.

Fixes #641

.github/workflows/on_pull_request.yaml

@@ -3,19 +3,24 @@ name: On Pull Request
 # On pull_request, we:
 # * always publish to charmhub at latest/edge/branchname
 # * always run tests
-
+# * always run builds with cache
 on:
   pull_request:
 
 jobs:
+  get-paths-and-build:
+    name: Get charm paths and build with cache
+    uses: canonical/charmed-kubeflow-workflows/.github/workflows/get_charms_build_with_cache.yaml@KF-6690-build-with-cache
 
   tests:
     name: Run Tests
+    needs: [get-paths-and-build]
     uses: ./.github/workflows/integrate.yaml
     secrets: inherit
 
   # publish runs in parallel with tests, as we always publish in this situation
   publish-charm:
     name: Publish Charm
+    needs: [get-paths-and-build]
     uses: ./.github/workflows/publish.yaml
     secrets: inherit