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.

[RytoEX]

Unfortunately, this seems to be a necessary evil until #10934 lands, unless we switch to larger (paid) runners. Since #10934 won't land for the 32.0.x branch, this PR has to land now to ensure Flatpak jobs can succeed in the interim. We can consider reverting this afterward, as it does add about a minute of extra time to the Flatpak jobs, and these are already the longest running jobs on CI.