Cherry-pick #22874 to 7.x: use environment variables in install-service.ps1 template

[leehinman]

Cherry-pick of PR #22874 to 7.x branch. Original message:

What does this PR do?

Replaces C:\ProgramData with environment variable env:PROGRAMDATA in
the install-service.ps1 scripts.

Why is it important?

Follows best practice for determining locations on Windows.

Checklist

• My code follows the style guidelines of this project
  - [ ] I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

run install-service.ps1 on Windows machine

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

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Build stats

• Build Cause: Pull request #22900 opened

• Start Time: 2020-12-03T17:08:13.295+0000

• Duration: 62 min 26 sec

Test stats 🧪

Test	Results
Failed	0
Passed	16829
Skipped	1398
Total	18227

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	16829
Skipped	1398
Total	18227