Run invoker plugin with a single thread to avoid build failures

Running the invoker plugin with multiple threads against an empty
Maven cache results in strange build failures where Maven claims that
it cannot find a jar or pom file for an artifact that it should be
able to find. It would appear that Maven is unable to cope with
concurrent writes to its cache.

This commit removes the usage of multiple threads that was introduced
in 4e907f19.

Fixes gh-2389

spring-boot-tools/spring-boot-loader/pom.xml

@@ -62,7 +62,6 @@
 							<addTestClassPath>true</addTestClassPath>
 							<skipInvocation>${skipTests}</skipInvocation>
 							<streamLogs>true</streamLogs>
-							<parallelThreads>4</parallelThreads>
 						</configuration>
 						<executions>
 							<execution>