CI: Prepare more space for Flatpak jobs

[tytan652]

Description

As it is we get ~16GB of space to build, without #10934 we need to ensure that at least 25GB is available at the price of 3-5 minutes of in-container deletion.

Based on https://github.com/obsproject/obs-deps-buildstream/blob/master/.github/actions/prepare-build-space/action.yaml but stripped down and adapted to in container env.

Motivation and Context

Fix Flatpak jobs until #10934

How Has This Been Tested?

PR on my fork to trigger CI.

Types of changes

• CI

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.