* Cache dependencies on CI workflow

* Use available fontforge version for CI action
Avoid a dirty OS upgrade to get a newer version of FontForge,
use what is available at Ubuntu 20.04

* Upgrade CI Workflow
By using an appimage of FontForge, it's easier to install the latest version, to cache it, and there is no extra dependencies clashes with  Ubuntu 20.04

* Make paths for fontforge absolute in makefile
It's necessary because fontforge is an AppImage and requires it

* Improve cache step on CI

wget don't download a duplicate if file already exists

* Generate base64 ascii encoded debug keystore

That can be used to transfer the keystore to a Github Secret

* Restore debug.keystore from github secrets

Get the asc encoded keystore from github secrets, 
and decode it back to a bynary file inside the CI run.

* Cleanup redundant lines and add explanation comment

* runs-on  ubuntu-latest

Co-authored-by: Jules Aguillon <jules@j3s.fr>

* add *.keystore.asc to .gitignore

* Clean up lines, adjust documentation

* use CURDIR automatic makefile variable

Co-authored-by: Jules Aguillon <jules@j3s.fr>

The Play Store now requires the "Signing Scheme V2", which is
implemented by apksigner.

The ant scripts included in the android SDK were removed recently. The
alternative is Gradle.
Gradle doesn't work well in this app because it's not possible to use
Nix to pull dependencies. Gradle will try to patch the SDK.
Also, it's very slow.

It turns out the required build script is quite simple.