improv/zipFilename: Fix cli zip filename contains .something issue

[angyonghaseyo]

Fix websites with .sg to have .zip as well

• [x] I've kept this PR as small as possible (~500 lines) by splitting it into PRs with manageable chunks of code
• [x] I've requested reviews from 1 reviewer
• [x] I've tested existing features (website scan, sitemap, custom flow) in both node index and cli
• [x] I've synced this fork with GovTechSG repo
• [ ] I've added/updated unit tests
• [ ] I've added/updated any necessary dependencies in package[-lock].json npm audit, portable installation on GitHub Actions