Fixing slim attacher CLI with requested agent version for download

[eyalkoren]

There seems to be a regression that is specifically related to running the slim attacher CLI jar with a requested --download-agent-version and without --agent-jar.
In order to reproduce, change manually the statically specified download version to anything other than null (e.g. latest) and run one of the cli-supporting integration tests, like Tomcat. As long as this field is set to null, these integration tests run with a specified --agent-jar, which works around the issue.

The error is:

Exception in thread "main" java.lang.ExceptionInInitializerError
	at co.elastic.apm.attach.ElasticApmAttacher.getBundledAgentJarFile(ElasticApmAttacher.java:199)
	at co.elastic.apm.attach.AgentAttacher$Arguments.<init>(AgentAttacher.java:384)
	at co.elastic.apm.attach.AgentAttacher$Arguments.parse(AgentAttacher.java:510)
	at co.elastic.apm.attach.AgentAttacher.main(AgentAttacher.java:108)
Caused by: java.lang.IllegalStateException: unable to get packaged agent within attacher jar, make sure to execute 'mvn clean package' first.
	at co.elastic.apm.attach.ElasticApmAttacher$AgentJarFileHolder.getAgentJarFile(ElasticApmAttacher.java:216)
	at co.elastic.apm.attach.ElasticApmAttacher$AgentJarFileHolder.<init>(ElasticApmAttacher.java:206)
	at co.elastic.apm.attach.ElasticApmAttacher$AgentJarFileHolder.<clinit>(ElasticApmAttacher.java:203)
	... 4 more

Consider changing the error message - make sure to execute 'mvn clean package' first, which is irrelevant to attacher users.

[apmmachine]

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• Start Time: 2023-04-06T11:28:22.880+0000

• Duration: 13 min 1 sec

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[SylvainJuge]

LGTM, tested manually.