fix: add newline to digestfile

[d3spairx]

What type of PR is this?

/kind other

What this PR does / why we need it:

This PR adds newline to buildah push --digestfile output file.

How to verify it

buildah push --digestfile digest.txt
cat -A digest.txt
# Should contain $ symbol at the end of the digest

Which issue(s) this PR fixes:

Special notes for your reviewer:

Smoke test job fails on pr-should-include-tests script. Since this change is pretty minor I'm not sure if it need any additional testing.

Does this PR introduce a user-facing change?

Added newline to buildah push --digestfile output file.

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[nalind]

I always imagined --digestfile as a complement to --iidfile, and we don't include a newline at the end of those because docker build doesn't. What's the intent behind making this behave differently?

[github-actions[bot]]

A friendly reminder that this PR had no activity for 30 days.

[d3spairx]

@nalind when we open digestfile in the terminal, we have broken terminal output because of the lack of newline. That is the whole intention of this PR.

[github-actions[bot]]

A friendly reminder that this PR had no activity for 30 days.

[TomSweeneyRedHat]

@d3spairx tests need help, and it would probably be good to rebase this.