Forge Run Configs don't place built mod in classpath

[NickAcPT]

Please include the following information in all issues:

• 2018.2-1.2.10-113
• IntelliJ IDEA 2018.2.4. Build #IU-182.4505.22, built on September 18, 2018
• Windows 10 1803 (OS Build 17134.345)
• Forge 1.12.2

When building the project using the created run configuration (a.k.a. gradle build), and then running the project with the provided configuration (GradleStart), forge launches fine, but it doesn't recognize the mod. Output files are going to \build\libs, but that directory isn't on the classpath somehow.