Duplicate builds should be a no-op

[tylfin]

Description

We'd like to rely on a pull model instead of kicking off the ad hoc job to build the most recent builds. To facilitate this, we changed the CRON to every hour:

rstudio-docker-products/.github/workflows/build-bake-preview.yaml

Line 3 in 0ca9449

- cron: '0 * * * *'

I think this is using more Github credits than it ought to, so instead of reverting the change, it would be nice to ensure duplicate builds exit as early as possible.