Add skipSave parameter

[andrewradamis-paay]

Add skipSave parameter. When set to true, this will skip updating the cache at the end of the step. This is useful to me in a pipeline I'm working on where I'm using this plugin for both caching and moving files between nodes. A simplified example below.

Additionally, this change should resolve #150

pipeline {
    agent 'build'

    stages {
        stage('Init') {
            steps {
                echo 'Initialize Terraform'
                cache(caches: [arbitraryFileCache(cacheName: "dot_terraform", path: '.terraform/')]) {
                    sh(script: 'terraform init -backend=false') // Initialize Terraform without backend configuration, downloads providers, modules, etc
                }
            }
        }
        stage('Deploy to Multiple Regions') {
            parallel {
                stage('us-east-1') {
                    agent {
                        label 'us-east-1'
                    }
                    steps {
                        cache(skipSave: true, caches: [arbitraryFileCache(cacheName: "dot_terraform", path: '.terraform/')]) {
                            sh(script: 'terraform init') // plan, deploy, etc
                        }
                    }
                }
                stage('us-west-2') {
                    agent {
                        label 'us-west-2'
                    }
                    steps {
                        cache(skipSave: true, caches: [arbitraryFileCache(cacheName: "dot_terraform", path: '.terraform/')]) {
                            sh(script: 'terraform init') // plan, deploy, etc
                        }
                    }
                }
            }
        }
    }
}

Testing done

I've added a basic unit test for this change, as well as tested it manually.

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[repolevedavaj]

Thank you for your contribution!