ci: the linkcheck job doesn't have to rebuild everything

The linkcheck step can be considerabily sped up by simply running the linkcheck executable on the pandocs build generated (and available as artifact) in the previous step: https://github.com/gbdev/pandocs/blob/master/.github/workflows/build-and-deploy.yml#L77

Open .github/workflows/build-and-deploy.yml around line 77 and inspect how the linkcheck step currently builds its input. Use the pandocs build artifact from the previous step instead, then verify the workflow runs linkcheck without rebuilding the documentation and still completes successfully.

Written by the indexing model from the issue text.