Use Environment.MachineName to get HostName

[gregkalapos]

Solves #1504

Other agents seem to do similar things based on elastic/apm#517 what Environment.MachineName does internally.

On Windows Environment.MachineName ends up in GetComputerNameW, on Unix it uses gethostname.

[apmmachine]

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• Start Time: 2021-10-04T16:04:46.027+0000

• Duration: 60 min 20 sec

• Commit: 60d8542

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Test	Results
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Skipped	126
Total	19851

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