Add Flatpak aarch64 builds (Part 1: build)

[tytan652]

Description

Add aarch64 support to the manifest and to the CI with a different approach from #8105.

[!IMPORTANT]
Publish workflow will be worked on once the PR design for build is approved.

[!CAUTION] This PR does not mean that RPis (or alike) will be supported, like x86_64 machines not all of them can handle OBS Studio.

About aarch64 on CI:

Building through QEMU emulation is really slow. So to avoid timing out the job (6h), the original job is split in two.

The first job build dependencies (4-5h) and creates an artifact, if the cache already exist the build is skipped.

And the second job build CEF and OBS Studio by relying on the cache, if no cache on the artifact of the previous job (2-3h without cache).

aarch64 builds are not enabled by default on the main workflow on PRs.

Motivation and Context

Publish Flatpak aarch64 builds.

How Has This Been Tested?

CI with a PR on my fork: https://github.com/tytan652/obs-studio/pull/16 (aarch64 artifacts availlable)

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

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