Updates custom-builder action to main branch

Updates the `uses` path for the custom-builder action in the MegaLinter custom flavor builder workflow from the `alpha` tag to the `main` branch. This ensures that the workflow uses the most up-to-date version of the custom builder action.

Author: nvuillam

mega-linter-runner/generators/mega-linter-custom-flavor/templates/megalinter-custom-flavor-builder.yml

@@ -66,7 +66,7 @@ jobs:
         continue-on-error: true
 
       - name: Build MegaLinter Custom Flavor
-        uses: oxsecurity/megalinter/flavors/custom-builder@alpha
+        uses: oxsecurity/megalinter/flavors/custom-builder@main
         with:
           megalinter-custom-flavor-builder-tag: ${{ github.event_name == 'release' && github.event.release.tag_name || 'alpha' }}
           is-latest: ${{ github.event_name == 'release' && 'true' || 'false' }}