Remove hardcoded Windows env var for JAVA_HOME

[brunoborges]

Issue describing the changes in this PR

This fixes #421. The bug described in #421 also affects running Azure Functions on Linux and on Windows Subsystem for Linux, besides macOS.

Pull request checklist

• [x] My changes do not require documentation changes
  • [ ] Otherwise: Documentation issue linked to PR
• [ ] My changes should not be added to the release notes for the next release
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