GHA-219 Fix notify-slack failing on runners without system Python (#127)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nils-werner-sonarsource

notify-slack/action.yml

@@ -44,9 +44,15 @@ runs:
       with:
         secrets: |
           development/kv/data/slack token | SLACK_TOKEN;
+    - name: Set up Python
+      uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5.6.0
+      with:
+        python-version: '3.10'
     - name: Install dependencies
       shell: bash
-      run: pip install -r "$GITHUB_ACTION_PATH/requirements.txt" --quiet
+      run: |
+        python3 -m venv "$GITHUB_ACTION_PATH/.venv"
+        "$GITHUB_ACTION_PATH/.venv/bin/pip" install -r "$GITHUB_ACTION_PATH/requirements.txt" --quiet
     - name: Send Slack Notification
       shell: bash
       env:
@@ -57,7 +63,7 @@ runs:
         INPUT_COLOR: ${{ inputs.color }}
         INPUT_MESSAGE: ${{ inputs.message != '' && inputs.message || format('*Run:* {0}/{1}/actions/runs/{2}\n*Failed Jobs:* {3}', github.server_url, github.repository, github.run_id, steps.collect_failed_jobs.outputs.failed-jobs) }}
       run: |
-        python "$GITHUB_ACTION_PATH/notify_slack.py" \
+        "$GITHUB_ACTION_PATH/.venv/bin/python" "$GITHUB_ACTION_PATH/notify_slack.py" \
           --channel "$INPUT_CHANNEL" \
           --project-name "$INPUT_PROJECT_NAME" \
           --icon "$INPUT_ICON" \