RFR: 8300099: Configuration fails to auto-detect build user through $USER in dockers

[Jie Fu]

Hi all,

Configuration is broken in dockers after JDK-8296478.
This is because `$USER` is not defined in dockers [1].

The fix will check `whoami` if we get an empty `$USERNAME`.

Thanks.
Best regards,
Jie

[1] https://stackoverflow.com/questions/54411218/docker-why-isnt-user-environment-variable-set

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Commit messages:
 - 8300099: Configuration is broken in dockers after JDK-8296478

Changes: https://git.openjdk.org/jdk/pull/11983/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11983&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300099
  Stats: 8 lines in 2 files changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11983.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11983/head:pull/11983

PR: https://git.openjdk.org/jdk/pull/11983