Fix CI fail on initial push of newly created extension

[s-weigand]

First of all thanks for the cookiecutter template ❤️

Sadly it doesn't always work out of the box.

When creating a new extension and choosing has_binder [n]: y the initial push fails due to check-manifest complaining about the binder folder not being in the MANIFEST.in.
Since the binder settings aren't needed for the extension itself, this can be fixed by excluding them from packaging.

After fixing this there is an error caused by popd (directory stack empty) since pushd wasn't used in the workflow.

After removing popd from the workflow template a freshly created extension passes the CI.

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[blink1073]

Thank you!

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