The ‘build’ keyword has been replaced by 'override-build’
introduced in snapcraft 2.41
The build scriptlet was originally introduced as a way to override the default build step with one’s own logic.
However, as part of an effort to add support for Snapcraft to have this for all lifecycle steps (i.e. not just build), a new scriptlet has been added that works similarly called override-build.
The biggest difference is that you can now call snapcraftctl build to run the default build step.
See Deprecation notices for further announcements.