Fix gallery build for prod

[jjspace]

Description

Small oversight in #12904 that the gallery was not getting built into the Build/Sandcastle2 directory when in PROD. This is needed for the website deployments. Small edit to the build files to make sure it's included.

I tested this by deploying to dev-sandcastle.cesium.com from this branch and the gallery showed up again.

Issue number and link

No issue

Testing plan

• run git clean -dxf
• run npm run build and npm run build-ts
• run PROD=true npm run build-sandcastle
• Verify that the Build/Sandcastle2/gallery directory exists and has all the gallery files.
• Extra points if you run cd Build/Sandcastle2 && npx http-server to test an isolated local server mimicking prod

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjspace!

✅ We can confirm we have a CLA on file for you.

[jjspace]

Opened the PR mostly for bookkeeping. I've already tested that the change works for dev-sandcastle.cesium.com so I'm going to merge to fix the prod build.

FYI @ggetz