Build Gatsby and Sanity projects based on changed files

[kristianfreeman]

This PR updates the GitHub Actions workflow to:

• Build the Sanity project/schema when changes occur in the sanity directory
• Builds the Gatsby project when things change anywhere else in the project (not in the sanity directory)

Example working Gatsby build: https://github.com/cloudflare/built-with-workers/actions/runs/32660067 Example working Sanity build: https://github.com/cloudflare/built-with-workers/actions/runs/32660072

Closes #8

[kristianfreeman]

Looks like there's a stuck build here or something? Merging ✌️