[core] Fix system properties being immutable (#4447)

When WorkflowSettings was constructed from an existing instance, as is
the case when we deserialize from an external configuration file, it
would result in an error:

Caused by: java.lang.UnsupportedOperationException
        at com.google.common.collect.ImmutableMap.put(ImmutableMap.java:450)
        at org.openapitools.codegen.config.WorkflowSettings$Builder.withSystemProperty(WorkflowSettings.java:465)

This was due to an error in `newBuilder(WorkflowSettings copy)` which
assigned builder.systemProperties with an immutable map. This is
incorrect because everything in the builder should be mutable until
.build() is invoked.

This likely affects CLI/Maven plugin as well for version 4.1.1 through 4.2.0.

modules/openapi-generator-gradle-plugin/samples/local-spec/README.md

@@ -11,6 +11,7 @@ gradle openApiGenerate
 gradle openApiMeta
 gradle openApiValidate
 gradle buildGoSdk
+gradle buildDotnetSdk
 gradle generateGoWithInvalidSpec
 ```
 

modules/openapi-generator-gradle-plugin/samples/local-spec/build.gradle

@@ -65,6 +65,20 @@ task buildGoSdk(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTas
     ]
 }
 
+task buildDotnetSdk(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTask){
+    generatorName = "csharp-netcore"
+    inputSpec = "$rootDir/petstore-v3.0.yaml".toString()
+    additionalProperties = [
+            packageGuid: "{321C8C3F-0156-40C1-AE42-D59761FB9B6C}",
+            useCompareNetObjects: "true"
+    ]
+    outputDir = "$buildDir/csharp-netcore".toString()
+    systemProperties = [
+            models: "",
+            apis  : "",
+    ]
+}
+
 task generateGoWithInvalidSpec(type: org.openapitools.generator.gradle.plugin.tasks.GenerateTask){
     validateSpec = true
     generatorName = "go"