move shadow jar buildscript to flink convention plugin

theodolite-benchmarks/build.gradle

-// Inherited to all subprojects
-// Make shadow plugin available, used for flink
-buildscript {
-  repositories {
-    maven {
-      url "https://plugins.gradle.org/m2/"
-    }
-  }
-  dependencies {
-    classpath "com.github.jengelman.gradle.plugins:shadow:6.0.0"
-  }
-}
-
 // Plugins
 allprojects {
   apply plugin: 'eclipse'

theodolite-benchmarks/buildSrc/build.gradle

+buildscript {
+  repositories {
+    maven {
+      url "https://plugins.gradle.org/m2/"
+    }
+  }
+  dependencies {
+    classpath "com.github.jengelman.gradle.plugins:shadow:6.0.0"
+  }
+}
+
 // to discover the precompiled script plugins
 plugins {
     id 'groovy-gradle-plugin'
@@ -9,4 +20,5 @@ repositories {
 
 dependencies {
     implementation 'gradle.plugin.com.github.spotbugs.snom:spotbugs-gradle-plugin:4.6.0'
+    implementation 'com.github.jengelman.gradle.plugins:shadow:6.0.0'
 }