buildscript: add overmind maven repository as a redundancy for forge repository

At some point original forge repository was replaced with overmind in gradle plugin, however GTNH one still contains forge repositories at the core. This should fix any issues arising from forge repositories missing artifacts.

build.gradle

@@ -186,6 +186,10 @@ configurations {
 }
 
 repositories {
+    maven {
+        name = "Overmind forge repo mirror"
+        url = "https://gregtech.overminddl1.com/"
+    }
     if(usesMixins.toBoolean()) {
         maven {
             name = "sponge"