jenkins: add workaround for groovy script & Java 11+

The Matrix Groovy Execution Strategy plugin appears to have an issue
on Java 11+ where string parameters are appearing as byte arrays
instead of strings. Implement a workaround to detect this and convert
the byte array back into a string.

Refs: jenkinsci/matrix-groovy-execution-strategy-plugin#20

jenkins/scripts/VersionSelectorScript.groovy

@@ -158,7 +158,12 @@ def canBuild = { nodeVersion, builderLabel, buildType ->
 
 int nodeMajorVersion = -1
 if (parameters['NODEJS_MAJOR_VERSION'])
-  nodeMajorVersion = parameters['NODEJS_MAJOR_VERSION'].toString().toInteger()
+  // Workaround for issue with Matrix Groovy Execution Strategy and Java 11+.
+  // https://github.com/jenkinsci/matrix-groovy-execution-strategy-plugin/issues/20
+  if (parameters['NODEJS_MAJOR_VERSION'].getClass().isArray())
+    nodeMajorVersion = new String(parameters['NODEJS_MAJOR_VERSION']).toInteger()
+  else
+    nodeMajorVersion = parameters['NODEJS_MAJOR_VERSION'].toString().toInteger()
 println "Node.js major version: $nodeMajorVersion"
 println "Node.js version: ${parameters['NODEJS_VERSION']}"