Fix runtime attach for current user

[SylvainJuge]

What does this PR do?

When using runtime attach as root user, it seems that in some circumstances the attach process hangs likely due to a sudo interactive input. In this case, the target JVM also runs as root, thus trying to use sudo is not even required at all.

When this happens with agent 1.23.0, the following stack trace is visible in a thread dump:

   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(java.base@11.0.9.1/Native Method)
        - waiting on <0x00000000c65e0ab0> (a java.lang.ProcessImpl)
        at java.lang.Object.wait(java.base@11.0.9.1/Object.java:328)
        at java.lang.ProcessImpl.waitFor(java.base@11.0.9.1/ProcessImpl.java:495)
        - waiting to re-lock in wait() <0x00000000c65e0ab0> (a java.lang.ProcessImpl)
        at co.elastic.apm.attach.UserRegistry$User.canSwitchToUser(UserRegistry.java:153)
        at co.elastic.apm.attach.UserRegistry$User.of(UserRegistry.java:144)
        at co.elastic.apm.attach.UserRegistry$User.access$100(UserRegistry.java:130)
        at co.elastic.apm.attach.UserRegistry.get(UserRegistry.java:125)
        at co.elastic.apm.attach.UserRegistry.getCurrentUser(UserRegistry.java:90)
        at co.elastic.apm.attach.AgentAttacher.<init>(AgentAttacher.java:73)
        at co.elastic.apm.attach.AgentAttacher.main(AgentAttacher.java:131)

What this PR does (at least as time of writing) is just making sure that we do not use sudo whenever the current user is already the one from the target JVM. Fixing the blocking behavior will be a second step.

Checklist

• ensure that we redirect IO in case the underlying command use stdin/stderr (would have allowed to debug this easily).
• adding a timeout when waiting for sudo execution to prevent such blocking behavior + properly log the issue
  --> not doable as waitFor with timeout variant has been added in Java 8.
• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

[ghost]

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Build stats

• Build Cause: Pull request #1819 updated

• Start Time: 2021-05-20T14:53:22.294+0000

• Duration: 58 min 32 sec

• Commit: 91e3d5d

Test stats 🧪

Test	Results
Failed	0
Passed	2133
Skipped	18
Total	2151

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Test	Results
Failed	0
Passed	2133
Skipped	18
Total	2151