Fixed stable stamping of wheels

[UebelAndre]

PR Checklist

Please check if your PR fulfills the following requirements:

• Does not include precompiled binaries, eg. .par files. See CONTRIBUTING.md for info
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Currently stable stamps do not make their way into wheels, this is the result of a copy/paste error in #554

Issue Number: N/A

What is the new behavior?

This change correctly wires up stable status stamp files for stamping py_wheel targets.

Does this PR introduce a breaking change?

• Yes
• No

Other information