Pester: Install latest version on WMF builds and skip installation if version is new enough for Azure DevOps which now ships with Pester

[bergmeister]

PR Summary

This has the following benefits:

• Azure DevOps images now ship with Pester by default, therefore we will be able to skip the installation step and rely on them bumping the Pester version. It saves around 8 seconds on Ubuntu and 20 seconds on Windows
• The WMF build will always have the latest version of Pester
• no more manual changes :-)

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[bergmeister]

Did some analysis and it seems between Pester 5.0.4 and 5.1.0 there is a breaking change for PowerShell v4. I opened an issue here: pester/Pester#1831