Install twine only when needed [build wheel]

[misl6]

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

Fixes our RPi wheels workflow (which is broken since a while), as we're installing twine along with the other pip dependencies and twine requires cryptography, which is currently unavailable on piwheels as requires a rust compiler on their runners.

Considering we're also installing twine at the appropriate time (see below), there's no need to keep a duplicated installation.

kivy/.ci/ubuntu_ci.sh

Line 300 in e932a4e

python3 -m pip install twine

kivy/.ci/windows_ci.ps1

Line 133 in e932a4e

python -m pip install twine