Fix Gradle 9 compatibility for task classes extending abstract types

- Make PluginInstallTask abstract (extends Sync)
- Make PluginPackageTask abstract (extends Zip)

In Gradle 8+/9, tasks extending abstract task types like Sync and Zip
must be declared as abstract classes. This allows Gradle to create
concrete implementations at runtime with proper dependency injection.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: pditommaso

src/main/groovy/io/nextflow/gradle/PluginInstallTask.groovy

@@ -1,13 +1,13 @@
 package io.nextflow.gradle
 
-
 import org.gradle.api.tasks.Sync
 
 /**
  * Gradle task which 'installs' (copies) this Nextflow plugin into
  * the local Nextflow plugins dir.
  */
-class PluginInstallTask extends Sync {
+abstract class PluginInstallTask extends Sync {
+
     PluginInstallTask() {
         group = 'Nextflow Plugin'
         description = 'Install this plugin into your local Nextflow plugins dir'

src/main/groovy/io/nextflow/gradle/PluginPackageTask.groovy

@@ -8,7 +8,7 @@ import org.gradle.api.tasks.bundling.Zip
 /**
  * Gradle task to package a Nextflow plugin into a zip file
  */
-class PluginPackageTask extends Zip {
+abstract class PluginPackageTask extends Zip {
 
     PluginPackageTask() {
         group = 'Nextflow Plugin'