Write chart_version only if latest_tag is defined

[fhofherr]

The chart_version output was written regardless of whether the latest_tag variable was defined. The latest_tag variable is only defined if skip_packaging is not set. This leads to issue #171.

Setting skip_packaging implies that an external process takes care of packaging the chart and setting the chart version. There is thus no need to write the chart_version output as it should already be known.

Closes #171

[IGx89]

@davidkarlsen would you be able to take a look at this PR too? It fixes a regression in v1.6 that's been preventing a lot of us from upgrading from v1.5.

[ChengjieLi28]

@fhofherr Hi. Thanks for your fix. Since there has been no official response for a long time, I will directly use the branch of your forked repo as the github action. This problem can be resolved via your branch. Please don't delete this branch, thank you. 😂😂🤷🏻‍♀️🤷🏻‍♀️