MK8S-25: Add clean step before build in GitHub Actions

Fix cleanup issues by running './doit.sh clean' before the main build:
- Ensures all clean functions are called, including index.html file cleanup
- Prevents directory removal failures caused by leftover index.html files
- Provides consistent clean state for each build in CI/CD
- Addresses root cause of packaging cleanup failures in e2e tests

This relies on doit's built-in clean mechanism to properly coordinate
cleanup across all tasks and their dependencies.

Author: rdebay-scality

.github/workflows/build.yaml

@@ -81,6 +81,8 @@ jobs:
           fetch-depth: 0
       - name: Set safe directory (since container is root and not user 1001)
         run: git config --global --add safe.directory ${{ github.workspace }}
+      - name: Clean previous build artifacts
+        run: cd "${{ github.workspace }}" && ./doit.sh clean
       - name: Build everything
         run: cd "${{ github.workspace }}" && ./doit.sh -n 4 --verbosity 2 --failure-verbosity 2
       - name: Prepare artifacts