Run debug builds on master branch pushes

[runningcode]

Summary

This runs debug builds on master branch pushes in order to populate the
github actions cache via the Gradle build action for faster PR builds. Future no-op builds should be significantly faster!
By default the Gradle build action writes to the caches on master branch builds. Other branches read from the master branch cache or their own branch cache. Here is more information on branch caching.
I'm assuming that since this is an OSS project the github action runners
are free so there is no cost to this.

I also renamed pr_build to build to reflect this change but that means that the required status check has been renamed.

[jpelgrom]

This action also generates debug apk files and uploads them as an artifact, which we don't need. The entire workflow is named pull request and also does lint checks which are unnecessary after merging.

Maybe some resources could be saved by just creating a new, simple job in the master push workflow?

I'm assuming that since this is an OSS project the github action runners are free so there is no cost to this.

I believe so but that doesn't mean we should do more work that isn't used