update integration CI to preserve charmcraft's cache between builds

.github/workflows/integrate.yaml

@@ -45,6 +45,7 @@ jobs:
     name: Integration Test
     runs-on: ubuntu-20.04
     strategy:
+      fail-fast: false
       matrix:
         microk8s-versions:
           - 1.25-strict/stable
@@ -53,6 +54,8 @@ jobs:
           - integration
           - integration-tls-provider
           - integration-tls-secret
+    env:
+      CRAFT_SHARED_CACHE: /home/runner/snap/charmcraft/common/cache/charmcraft/
     steps:
       - name: Check out repo
         uses: actions/checkout@v4
@@ -66,12 +69,37 @@ jobs:
           juju-channel: 3.1/stable
           charmcraft-channel: latest/candidate
 
+      - name: Setup Charmcraft's cache
+        id: cache
+        uses: actions/cache@v4
+        with:
+          path: ${{ env.CRAFT_SHARED_CACHE }}
+          # Cache keys must be unique - there is no overwrite mechanic.  Add IDs to avoid this (is there a better set of IDs?)
+          # partial ref: https://github.com/actions/cache/blob/main/tips-and-workarounds.md#update-a-cache
+          # To match the most recent previous cache, use restore-keys with the craft-shared-cache prefix.  This will hit the
+          # "first" match, which will be from the most recent run.  
+          # ref: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#matching-a-cache-key
+          key: craft-shared-cache-${{ github.run_id }}-${{ github.run_attempt }}-${{ github.job }}-${{ strategy.job-index }}
+          restore-keys: craft-shared-cache
+
+      # DEBUG: remove this after testing
+      - name: emit charmcraft cache directory contents
+        run: |
+          find $CRAFT_SHARED_CACHE | wc -l
+          find $CRAFT_SHARED_CACHE/wheels | wc -l
+
       - name: Run integration tests
         run: |
           juju add-model test-istio
           tox -e ${{ matrix.integration-types }} -- --model test-istio
         timeout-minutes: 80
-
+
+      # DEBUG: remove this after testing
+      - name: emit charmcraft cache directory contents
+        run: |
+          find $CRAFT_SHARED_CACHE | wc -l
+          find $CRAFT_SHARED_CACHE/wheels | wc -l
+
       - name: Setup Debug Artifact Collection
         run: mkdir tmp
         if: failure()