java.util.jar.JarInputStream.getManifest() returns null for quarkus-runner.jar

[dabloem]

Describe the bug
JarInputStream.getManifest() returns null for quarkus-runner.jar

Expected behavior
Return non-null Manifest.class

Actual behavior
Returns null

To Reproduce
Steps to reproduce the behavior:

1. create quarkus-runner.jar (which includes META-INF/MANIFEST.MF)
2. execute the following code

InputStream file = Main.class.getResourceAsStream("/quarkus-runner.jar");
Manifest manifest = new JarInputStream(file).getManifest();
Objects.requireNonNull(manifest);

3. Verify NullPointerException

Environment (please complete the following information):

• Output of uname -a or ver: Linux
• Output of java -version: OpenJDK-8, IBM-8, Corretto-8
• Quarkus version or git rev: 1.0.0.CR1

Additional context
Issue encountered when using azure-webapp-maven-plugin:1.8.0, which verifies if the deployed jar is an executable jar based on JarInputStream.class

[geoand]

This seems related to: https://bugs.openjdk.java.net/browse/JDK-8031748

So basically for JarInputStream the manifest needs to be the first entry in the jar

[geoand]

#5409 should take care of this