Only run CI workflows for pull requests against main branch

[scottyhq]

#56 (comment)

Checks aren't necessary to merge, so can leave it as-is, but it looks like the preview workflow will run currently against any PR:

ICESat-2-Hackweek-2023/.github/workflows/netlifypreview.yaml

Lines 3 to 5 in 3b35909

	on:
	pull_request_target:
	types: [labeled, synchronize]