Speedup CI by bootstrapping PowerShell module installations in background and in parallel, whilst the .NET SDK is being installed

[bergmeister]

PR Summary

Most of the time is spent bootstrapping the .NET SDK, meaning the installation of other required PowerShell modules can be done in the background and in parallel. This reduces the build time by around 20 seconds. Due to indentation shift, please use a white-space ignoring diff.

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.

[rjmholt]

@JamesWTruher need you to review this one