[GHA] Better handling of environment variables and change detection
This should help reduce the number of times that Android AVD tests have to be run, along with consolidating some of the environment variable logic.
Verified that changelog detection is skipped with the most recent commit.
The ci.yml part got fixed in #1641, but the changelog.yml changes are totally cool.
@Arusekk do you approve of this?
I think I just accidentally'd that merge. DO NOT MERGE until I have a chance to look at this again
This also looks inferior to #1594, or I am missing something
Need to follow up on this one.