[ci] Gstring to string

Author: hazendaz

src/main/groovy/org/codehaus/mojo/spotbugs/SpotBugsGui.groovy

@@ -149,7 +149,7 @@ class SpotBugsGui extends AbstractMojo implements SpotBugsPluginsTrait {
 
             log.info('File Encoding is ' + effectiveEncoding.name())
 
-            sysproperty(key: "file.encoding" , value: effectiveEncoding.name())
+            sysproperty(key: 'file.encoding' , value: effectiveEncoding.name())
 
             // spotbugs assumes that multiple arguments (because of options) means text mode, so need to request gui explicitly
             jvmarg(value: '-Dfindbugs.launchUI=gui2')