Upload artifacts in parallel (address artifactorys "Maven Snapshot Version Behaviour") (#1524)

private/tools/java/com/github/bazelbuild/rules_jvm_external/maven/MavenPublisher.java

diff --git a/private/tools/java/com/github/bazelbuild/rules_jvm_external/maven/MavenPublisher.java b/private/tools/java/com/github/bazelbuild/rules_jvm_external/maven/MavenPublisher.java
index 74e3498..9b104d3 100644
--- a/private/tools/java/com/github/bazelbuild/rules_jvm_external/maven/MavenPublisher.java
+++ b/private/tools/java/com/github/bazelbuild/rules_jvm_external/maven/MavenPublisher.java
@@ -124,7 +124,9 @@
     SigningMetadata signingMetadata =
         new SigningMetadata(gpgSign, useInMemoryPgpKeys, signingKey, signingPassword);
 
-    final ExecutorService executorService = Executors.newSingleThreadExecutor();
+    final ExecutorService executorService = Executors.newFixedThreadPool(
+      Optional.ofNullable(System.getenv("RJE_MAX_THREADS")).map(Integer::parseInt).orElse(8)
+    );
 
     try {
       run(