Temp workaround: use a single thread to generate native image

Workaround for oracle/graal#1927

Should be possible to remove this in 19.3.1

This is a different problem to the previous one that this
fix worked around, however because we were using this setting
we only ran into the thread safety problem after it was removed.

core/deployment/src/main/java/io/quarkus/deployment/pkg/steps/NativeImageBuildStep.java

@@ -203,6 +203,11 @@ public NativeImageBuildItem build(NativeConfig nativeConfig, NativeImageSourceJa
             command.add("-H:InitialCollectionPolicy=com.oracle.svm.core.genscavenge.CollectionPolicy$BySpaceAndTime"); //the default collection policy results in full GC's 50% of the time
             command.add("-jar");
             command.add(runnerJarName);
+            //dynamic proxy generation is not thread safe
+            //should be fixed in 19.3.1
+            //but we need to add this option back to prevent intermittent failures
+            //https://github.com/oracle/graal/issues/1927
+            command.add("-J-Djava.util.concurrent.ForkJoinPool.common.parallelism=1");
             if (nativeConfig.enableFallbackImages) {
                 command.add("-H:FallbackThreshold=5");
             } else {