Set environment variable to avoid Get-AzAccessToken breaking change

[AnatoliB]

One of the next versions of Az.Accounts will have a breaking change: by default, the Get-AzAccessToken will return a SecureString instead a plaintext string even when the -AsSecureString switch is not provided. This environment variable will force the new Get-AzAccessToken to keep the old behavior by default.

Pull request checklist

• 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
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

[andystaples]

LGTM