[core] Fix system properties being immutable

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.

see #3815 (closed)

I'm tagging with Gradle Plugin because this is where it was noticed. This likely affects CLI/Maven plugin as well for version 4.1.1 through 4.2.0.

cc @OpenAPITools/generator-core-team

To test, build locally, then:

cd modules/openapi-generator-gradle-plugin
./gradlew publishToMavenLocal
cd samples/local-spec
gradle -PopenApiGeneratorVersion=4.2.1-SNAPSHOT buildDotnetSdk

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