Resurrect edge/candidate testing

[dimaqq]

Description

I'm disabling edge and candidate testing (2 workflows) because they are not useful:

always fails:
https://github.com/juju/python-libjuju/actions/workflows/test_edge.yaml

always succeeds with "no candidate to test":
https://github.com/juju/python-libjuju/actions/workflows/test_edge.yaml

the workflow is also buggy, I'll attach a wip diff.

one day, these tests need to be revived, though against 3.6 and not 3.1

Urgency

Casually wishlisting

Code I'd Like to Run

...

[dimaqq]

diff --git a/.github/workflows/test_edge.yaml b/.github/workflows/test_edge.yaml
index 27a949c..910479d 100644
--- a/.github/workflows/test_edge.yaml
+++ b/.github/workflows/test_edge.yaml
@@ -13,20 +13,27 @@ jobs:
       matrix:
         python:
           - "3.8"
-          - "3.9"
           - "3.10"
+          - "3.12"
+        juju:
+          - "3.1/edge"
+          - "3.2/edge"
+          - "3.3/edge"
+          - "3.4/edge"
+          - "3.5/edge"
+          - "3.6/edge"
     steps:
       - name: Download artifact from previous workflow
         id: download_artifact
-        uses: dawidd6/action-download-artifact@v2
+        uses: actions/download-artifact@v4
         with:
           workflow_conclusion: success
-          name: juju-last-edge-version
+          name: juju-last-edge-version-${{ matrix.python }}
           if_no_artifact_found: ignore
       - name: Check if there is a new candidate
         shell: bash
         run: |
-          edge=$(snap info juju | grep 3.1/edge | awk '{print $2}')
+          edge=$(snap info juju | grep $${ matrix.juju }} | awk '{print $2}')
           last_tested=NA
           if [ -f juju-last-edge-version ]; then
             last_tested=$(cat juju-last-edge-version)
@@ -34,7 +41,7 @@ jobs:
           echo "Last tested was $last_tested"
           echo "Latest juju version found is $edge"
           next_test=NA
-          if [[ "$edge" == "^" ]]; then
+          if [[ -z "$edge" ]]; then
             echo "No edge to test"
           else
             if [[ "$edge" == "$last_tested" ]]; then
@@ -59,7 +66,7 @@ jobs:
         uses: charmed-kubernetes/actions-operator@main
         with:
           provider: lxd
-          juju-channel: 3.4/edge
+          juju-channel: ${{ matrix.juju }}
       - name: Run integration
         if: ${{ env.next-test != 'NA' }}
         # Force one single concurrent test
@@ -68,5 +75,5 @@ jobs:
         if: ${{ env.next-test != 'NA' }}
         uses: actions/upload-artifact@v4
         with:
-          name: juju-last-edge-version
+          name: juju-last-edge-version-${{ matrix.python }}
           path: ~/juju-last-edge-version

[dimaqq]

current issue: tests are ran in parallel and step on each other's "juju-last-edge-version" artifact toes, upload fails when this key exists. arguably overwrite: true is needed, and per-matrix-combo unique key.