Fix logic on package-buildpack.sh

[dmikusa]

Summary

• The previous logic would require PUBLISH=true to run pack buildpack package and publish the result. This was OK, but if PUBLISH was set to anything else, even an empty string, it would still run pack buildpack package. It doesn't seem like this was actually happening in the pipelines, but this change makes the logic stricter. You need to have PUBLISH=true exactly, or it won't publish.
• Inside the publish block, we were checking if VERSION_MINOR and VERSION_MAJOR have content. This was incorrectly assuming that these variables will always be set, which was causing pipeline failures. This logic changes the behavior to account for them being not set or empty, and in both cases skips the block that would add additional tags. If there is content, then we'll set the additional tags.

Checklist

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