Manually set the artifactId for publishing

[xpple]

Transferred from FabricMC/fabric-example-mod#233.

By default, Gradle will use the project name configured in settings.gradle (rootProject.name), after which it will default to the project's directory name. The directory name is very often equal to the capitalised mod name. This can be problematic for GitHub Packages, since capitalised artifacts are not allowed. This PR prevents that from happening.

Edit: See this stackoverflow question for the error GitHub Packages returns.