GeoJSON source URL doesn't change in fork builds

[jacobwhall]

I forked this repository, and now I cannot use the included GitHub Action to preview changes before merging them upstream because index.html has the MapRVA repository hard-coded as a source. It would be nice if the build script fills in the repository name and owner into the source URL using environment variables, using MapRVA as a fallback.

[dschep]

fixed by ecdd971