[gh-actions] Add new testapp and upload artifacts...

[opacam]

So we can dowload the built testapp for each job we make, at gh-actions tab, (via a button on the top right after the job is completed)

In here we do:

• Add a new testapp for armeabi-v7a via a matrix
• Create artifacts for both testapps: arm64-v8a & armeabi-v7a
• Pin actions/checkout to v1 (because I started to get git failures with rebuild_updated_recipes.py script, and pinning to v1 solved the problem)

See also: The mentioned git error at my fork

[AndreMiras]

Awesome, well done!