Build QGreenland Core in the open

[mfisher87]

We currently build QGreenland Core (the actual zip package) on a private server, but we could be more transparent by e.g. building in AWS/Azure/whatever

Not to be confused with QGreenland's Docker images which are built in GitHub Actions. I think GitHub Actions may be a poor option for building Core because (a) GitHub limits artifact filesizes, (b) I don't know how users would access the artifact, (c) GitHub limits Actions runtime.