Allow users to configure Gradle build's current working directory

[dmikusa]

Describe the Enhancement

Presently, Gradle will build from the app root (i.e. /workspace). This isn't configurable. We should expose an env variable that makes this configurable, like BP_GRADLE_BUILD_WORKING_DIR. The user can then control the location from with gradle will be run.

We'll need to be careful that build commands are being run with full paths and that no relative paths are being used, otherwise, this might break some things.

Blocked by paketo-buildpacks/libbs#288

Motivation

Make the buildpack more configurable.