CI: Factorize Github labels checks

[tytan652]

Description

Factorize all 'Check for Github Labels' as one job so this step/job is done once.

It also fixes macOS and Ubuntu caches without dates when the CI is run outside of a PR.

Outputs is preferred over environment variable to store values.

NOTE: The use of fromJSON() is required to make GHA read the value as a boolean.

Motivation and Context

I'm working on aarch64 Flatpak CI. And seeing the same step being repeated multiple times, I ask myself if it could be factorized.

1 is less than 4, so it's better

So my work on Flatpak aarch64 will rely on this CI change.

How Has This Been Tested?

Tested on CI on my fork while I was working on Flatpak aarch64 with and without Seeking Testers label.

Types of changes

• CI Bug fix (non-breaking change which fixes an issue)

• CI Tweak (non-breaking change to improve existing 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.