Change behaviour of NO_PUSH to be boolean

[sgibson91]

I have been trying to use this variable in GitHub Actions like so:

NO_PUSH: ${{ github.event_name == 'pull_request' }}

i.e. a build triggered by a PR would not be pushed to a registry, but a build triggered by a push to main (or whatever other events are configured by the workflow) would be pushed, which I believe is a pretty common use case.

I was surprised that this didn't work and my builds were never pushed to a registry on either trigger, and I realised this is because the action only checks if NO_PUSH is not empty rather than treating it like a boolean. The only way to get the desired behaviour (without this PR or some other change) would be to have two workflow files that are identical other than the one triggered by a PR has NO_PUSH set. You can see this in the build.yaml and test.yaml files here: https://github.com/2i2c-org/hub-user-image-template/tree/main/.github/workflows

This PR changes the behaviour of NO_PUSH to be more boolean and allows us to achieve the desired workflow in a single workflow file and without duplicating code.

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