Improvements to the workflow

[derivativeoflog7]

I made a few changes I think can make compiling with GH Actions easier

• Add the ability to upload the keystore as a base64 encoded string in the repo secrets, to avoid having to upload a keystore and having forks mess with branches or commits to make test builds; while fallbacking to using the keystore in the repo if the secret is not set
• Defaulting USER_ID_OWNER to 0 if it's not set in the secrets (the feature is broken too as an int is too small to hold all possible user IDs, my own user ID breaks it) (why are there two features that have an override for the owner only anyway?)
• Skip downloading the language file if the secret is not set
• Add instructions