Jenkinsfile: switch to declarative pipeline

Jenkinsfile

@@ -1,39 +1,37 @@
 #!/usr/bin/env groovy
-if (env.BRANCH_NAME.startsWith('PR-')) {
-    properties([[$class: 'BuildDiscarderProperty', strategy: [$class: 'LogRotator', artifactNumToKeepStr: '1']]])
-} else {
-    properties([[$class: 'BuildDiscarderProperty', strategy: [$class: 'LogRotator', artifactDaysToKeepStr: '30', artifactNumToKeepStr: '5']]])
-}
 
-if (env.CHANGE_TARGET == 'master') {
-    error("This pull request targets the wrong branch. Please reopen the pull request targetting the dev branch.")
-}
-
-node('master') {
-    stage('Pull changes') {
-        checkout scm
+pipeline {
+    agent { label 'colobot-build' }
+    options {
+        buildDiscarder(logRotator(artifactDaysToKeepStr: '30', artifactNumToKeepStr: '5'))
     }
+    stages {
+        stage('Check pull request target') {
+            when { changeRequest() }
+            steps {
+                script {
+                    if (env.CHANGE_TARGET == 'master') {
+                        throw "This pull request targets the wrong branch. Please reopen the pull request targetting the dev branch."
+                    }
+                }
+            }
+        }
+        stage('Build data') {
+            steps {
+                sh '''
+                    cmake -DCMAKE_INSTALL_PREFIX=/install -DCOLOBOT_INSTALL_DATA_DIR=/install/data ..
+                    make
+                    rm -rf install
+                    DESTDIR=. make install
+                '''
+            }
+        }
 
-    stage('Build data') {
-        sh 'mkdir -p build'
-        dir('build') {
-            sh '''
-                cmake -DCMAKE_INSTALL_PREFIX=/install -DCOLOBOT_INSTALL_DATA_DIR=/install/data ..
-                make
-                rm -rf install
-                DESTDIR=. make install
-            '''
+        stage('Archive data') {
+            steps {
+                sh 'rm -f data.zip'
+                zip zipFile: 'data.zip', archive: true, dir: 'build/install'
+            }
         }
     }
-
-    stage('Archive data') {
-        sh 'rm -f data.zip'
-        zip zipFile: 'data.zip', archive: true, dir: 'build/install'
-    }
-
-    // Clean workspace after building pull requests
-    // to save disk space on the Jenkins host
-    if (env.BRANCH_NAME.startsWith('PR-')) {
-        cleanWs()
-    }
 }