RFR: 7903515: Some default env vars on Windows not set

[Christian Stein]

Please review this change to fix setting of some default environment variables on Windows.

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Commit messages:
 - Update comment
 - Update Tool.java
 - Update Tool.java

Changes: https://git.openjdk.org/jtreg/pull/166/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=166&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903515
  Stats: 26 lines in 2 files changed: 8 ins; 1 del; 17 mod
  Patch: https://git.openjdk.org/jtreg/pull/166.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/166/head:pull/166

PR: https://git.openjdk.org/jtreg/pull/166