Add clean instructions to be build cache-friendly

[gsmet]

The Gradle Enterprise build cache will only work when it is sure it will get a clean state, and for that, we need a clean.

This shouldn't slow down our build much even when the cache is disabled.

[quarkus-bot]

Failing Jobs - Building 1adadd4

Status	Name	Step	Failures	Logs	Raw logs
✖	JVM Tests - JDK 11	Build	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 17
✔️	JVM Tests - JDK 19
✔️	Maven Tests - JDK 11
⌛	Maven Tests - JDK 11 Windows	Build	⚠️ Check →	Logs	Raw logs
⌛	Native Tests - HTTP	Build	⚠️ Check →	Logs	Raw logs
⌛	Native Tests - Security1	Build	⚠️ Check →	Logs	Raw logs

Full information is available in the Build summary check run.

Failures

⚙️ JVM Tests - JDK 11 #

- Failing: extensions/hibernate-orm/deployment 
! Skipped: extensions/flyway/deployment extensions/hibernate-envers/deployment extensions/hibernate-reactive/deployment and 92 more

📦 extensions/hibernate-orm/deployment

✖ io.quarkus.hibernate.orm.multiplepersistenceunits.MultiplePersistenceUnitsCdiEntityManagerTest. - More details - Source on GitHub

java.lang.RuntimeException: java.lang.RuntimeException: Failed to start quarkus
	at io.quarkus.test.QuarkusUnitTest.beforeAll(QuarkusUnitTest.java:705)
	at org.junit.jupiter.engine.descriptor.ClassBasedTestDescriptor.lambda$invokeBeforeAllCallbacks$12(ClassBasedTestDescriptor.java:395)