Nice post. I didn't had the change to try it yet but I'll add this small change:
bintray {
user = properties.getProperty("bintray.user", System.getenv("bintrayUser"))
key = properties.getProperty("bintray.apikey", System.getenv("bintrayApiKey"))
...
}
This way I'm able to publish to bintray using local.properties credentials without creating a release (for some reason).
The fallback is GitHub Secrets :)