fix: use single build url upon creation (#4323)

[drewbo]

Changes proposed in this pull request:

• Implements the refactor in https://github.com/cloud-gov/pages-core/issues/4323#issuecomment-2444939054
• Closes #4323

Notes

• [x] Needs new tests
• [x] Verification on dev
• [x] Rewrite the url column on the build table using this logic

security considerations

None

[apburnes]

Running into a error in dev when trying to rebuild a site branch. Cannot destructure SiteBranchConfigs via utils/build.js

[drewbo]

Checking on dev, for all combination of preview, site, and domained urls:

• [x] Url appears correctly on frontend application
• [x] Url appears correctly on admin
• [x] Double check the url in the build response (via serializer)
• [x] Site receives the proper environment variables BASEURL and SITE_PREFIX
• [x] Github successful status checks show the url
• [x] Try an ASR for good measure