Reuse the build executor thread pool for parallel execution

[mkouba]

• for class transformation and ArC resource generation
• previously separate executor services were created and shut down

[mkouba]

@manovotn this intermittent failure might be related to #26968.

[quarkus-bot]

Failing Jobs - Building efe6397

Status	Name	Step	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 11
✔️	JVM Tests - JDK 17
✖	JVM Tests - JDK 18	Build	Failures	Logs	Raw logs

Full information is available in the Build summary check run.

Failures

⚙️ JVM Tests - JDK 18 #

- Failing: extensions/smallrye-reactive-messaging-kafka/deployment 
! Skipped: integration-tests/kafka-oauth-keycloak integration-tests/kafka-sasl-elytron integration-tests/kubernetes/quarkus-standard-way-kafka and 3 more

📦 extensions/smallrye-reactive-messaging-kafka/deployment

✖ io.quarkus.smallrye.reactivemessaging.kafka.deployment.testing.KafkaDevServicesContinuousTestingTestCase.testContinuousTestingScenario2 line 83 - More details - Source on GitHub

org.awaitility.core.ConditionTimeoutException: Failed to wait for test run 4 State{lastRun=3, running=true, inProgress=true, run=1, passed=0, failed=1, skipped=0, isBrokenOnly=false, isTestOutput=false, isInstrumentationBasedReload=false, isLiveReload=true}
	at io.quarkus.test.ContinuousTestingTestUtils.waitForNextCompletion(ContinuousTestingTestUtils.java:44)
	at io.quarkus.smallrye.reactivemessaging.kafka.deployment.testing.KafkaDevServicesContinuousTestingTestCase.testContinuousTestingScenario2(KafkaDevServicesContinuousTestingTestCase.java:83)

[mkouba]

I'm going to merge this PR. CI failures are not related.

[manovotn]

@manovotn this intermittent failure might be related to #26968.

This is true, I think the assertion can come too early since there can be a race condition.
PR incoming shortly