Resolving artifacts via gradle should respect $GRADLE_USER_HOME

[daphil19]

Currently, gradleHome looks in the user's home directory for .gradle. While this is the default place for the $GRADLE_USER_HOME env var, users can also configure it. Kotlin-Language-Server should first check for the presence of the env var and use that, falling back to the default location.

I imagine there is something similar we might need to check for with maven, but I haven't looked into what that would be.

When coupled with #255, this results in the kotlin compiler & stdlib not being found, resulting in a failed build.