Do not run checks on draft PRs

[vgeorge]

It would be nice if we could avoid running checks on draft PRs (like #613) and trigger only when the PR is ready for review.

cc @kylebarron

[kylebarron]

One interesting option is something like rouault/gdal@422220b where you can annotate a PR title with [no ci] to avoid running CI checks.

Sometimes I like having CI run on draft PRs. Even if they're not ready for review yet, it can be nice to see if CI passes