Auto merge of #126116 - Kobzol:ci-remove-setup-python, r=jdno

CI: remove `Setup Python` action

This action was added recently in rust-lang/rust#125590, but it shouldn't really be needed, as our CI was working fine before without it. Our base Ubuntu 20.04 images use Python 3.8, while this action installed Python 3.12, but we don't really need that.

Since this action does not support ARM yet, this blocks rust-lang/rust#126113. See rust-lang/rust#125590 (comment).

r? `@jdno`