Add workflow that signals rasterio

[sgillies]

So that rasterio will immediately build against GDAL release candidates and provide some feedback. Work on the rasterio side is here rasterio/rasterio#2967.

I'm not sure how this can be generalized to other projects. I suspect QGIS is doing something smarter and better, but I felt like it was good to learn about using these GitHub ways.

cc @rouault @dbaston @nyalldawson

Resolves #8688.

[coveralls]

coverage: 67.881% (+0.008%) from 67.873%
when pulling 40ec475 on sgillies:issue8688
into 170bc25 on OSGeo:master.

[rouault]

I'm OK with that. It seems however odd to me that github doesn't provide a more straightforward way of doing that which doesn't involve doing changes in the upstream repository. github should have in theory all the information needed so a downstream project can request being triggered by an event in another github repo...

[sgillies]

@rouault I expected that GitHub would have a better solution, and maybe they do in the form of installable actions. To allow one project to subscribe to events in any other public project would require a global event stream, yeah?

An alternative to this: write a rasterio cron job that used the GitHub API to search for new GDAL tags. Or implement a subscription to https://github.com/osgeo/gdal/tags.atom. I'll give that some thought. It's a little more work to track seen tags or otherwise detect new tags in the feed, but it would remove the need for secrets or a new GDAL workflow.

[sgillies]

Not needed!